compilation fixes for debian/wheezy/amd64: gfx/vg.c too few arguments to function; missing channel_layout.h

avutil 1.0.8 does not seem to include avutil/channel_layout.h, so conditionalize that

compilation on Debian wheezy amd64 is failing like so without the attached patch:

CC gfx/vg.c
gfx/vg.c: In function 'vg_set_nonblock_state':
gfx/vg.c:71:4: warning: passing argument 1 of 'vg->driver->swap_interval' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:71:4: note: expected 'void *' but argument is of type 'int'
gfx/vg.c:71:4: error: too few arguments to function 'vg->driver->swap_interval'
gfx/vg.c: In function 'vg_init':
gfx/vg.c:90:4: warning: passing argument 1 of 'gfx_ctx_init_first' makes pointer from integer without a cast [enabled by default]
In file included from gfx/vg.c:22:0:
gfx/gfx_context.h:123:25: note: expected 'void *' but argument is of type 'int'
gfx/vg.c:90:4: error: too few arguments to function 'gfx_ctx_init_first'
In file included from gfx/vg.c:22:0:
gfx/gfx_context.h:123:25: note: declared here
gfx/vg.c:98:4: error: too few arguments to function 'vg->driver->get_video_size'
gfx/vg.c:101:4: warning: passing argument 1 of 'vg->driver->swap_interval' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:101:4: note: expected 'void *' but argument is of type 'int'
gfx/vg.c:101:4: error: too few arguments to function 'vg->driver->swap_interval'
gfx/vg.c:102:4: error: too few arguments to function 'vg->driver->update_window_title'
gfx/vg.c:115:4: warning: passing argument 1 of 'vg->driver->set_video_mode' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:115:4: note: expected 'void *' but argument is of type 'unsigned int'
gfx/vg.c:115:4: error: too few arguments to function 'vg->driver->set_video_mode'
gfx/vg.c:121:4: error: too few arguments to function 'vg->driver->get_video_size'
gfx/vg.c:126:7: warning: passing argument 1 of 'vg->driver->translate_aspect' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:126:7: note: expected 'void *' but argument is of type 'uint32_t'
gfx/vg.c:126:7: error: too few arguments to function 'vg->driver->translate_aspect'
gfx/vg.c:138:4: warning: passing argument 2 of 'vg->driver->input_driver' from incompatible pointer type [enabled by default]
gfx/vg.c:138:4: note: expected 'const struct input_driver_t **' but argument is of type 'void **'
gfx/vg.c:138:4: error: too few arguments to function 'vg->driver->input_driver'
gfx/vg.c:163:4: warning: passing argument 1 of 'vg->driver->init_egl_image_buffer' discards 'const' qualifier from pointer target type [enabled by default]
gfx/vg.c:163:4: note: expected 'void *' but argument is of type 'const struct video_info_t *'
gfx/vg.c:163:4: error: too few arguments to function 'vg->driver->init_egl_image_buffer'
gfx/vg.c: In function 'vg_free':
gfx/vg.c:197:4: error: too few arguments to function 'vg->driver->destroy'
gfx/vg.c: In function 'vg_copy_frame':
gfx/vg.c:333:7: warning: passing argument 1 of 'vg->driver->write_egl_image' discards 'const' qualifier from pointer target type [enabled by default]
gfx/vg.c:333:7: note: expected 'void *' but argument is of type 'const void *'
gfx/vg.c:333:7: warning: passing argument 2 of 'vg->driver->write_egl_image' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:333:7: note: expected 'const void *' but argument is of type 'unsigned int'
gfx/vg.c:333:7: warning: passing argument 7 of 'vg->driver->write_egl_image' makes integer from pointer without a cast [enabled by default]
gfx/vg.c:333:7: note: expected 'unsigned int' but argument is of type 'void **'
gfx/vg.c:333:7: error: too few arguments to function 'vg->driver->write_egl_image'
gfx/vg.c: In function 'vg_frame':
gfx/vg.c:389:4: error: too few arguments to function 'vg->driver->update_window_title'
gfx/vg.c:392:4: error: too few arguments to function 'vg->driver->swap_buffers'
gfx/vg.c: In function 'vg_alive':
gfx/vg.c:404:10: warning: passing argument 3 of 'vg->driver->check_window' from incompatible pointer type [enabled by default]
gfx/vg.c:404:10: note: expected '_Bool *' but argument is of type 'uint32_t *'
gfx/vg.c:404:10: warning: passing argument 5 of 'vg->driver->check_window' makes pointer from integer without a cast [enabled by default]
gfx/vg.c:404:10: note: expected 'unsigned int *' but argument is of type 'unsigned int'
gfx/vg.c:404:10: error: too few arguments to function 'vg->driver->check_window'
gfx/vg.c: In function 'vg_focus':
gfx/vg.c:411:4: error: too few arguments to function 'vg->driver->has_focus'
gfx/vg.c:412:1: warning: control reaches end of non-void function [-Wreturn-type]
make: *** [obj-unix/gfx/vg.o] Error 1

gcc (Debian 4.7.2-5) 4.7.2