vala: update to vala 0.24

vala 0.16 had compile issues on Debian Unstable

error was:
checking for GLIB... no
configure: error: Package requirements (glib-2.0 >= 2.16.0 gobject-2.0 >= 2.16.0) were not met:

No package 'glib-2.0' found
No package 'gobject-2.0' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables GLIB_CFLAGS
and GLIB_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>