northbridge/intel/i945/Kconfig: Select VGA

Commit 0092c999 (i945: Support text mode gfx init) [1] broke building
the Lenovo X60 with native graphics initialization by selecting
`CONFIG_MAINBOARD_DO_NATIVE_VGA_INIT`.

        CC         northbridge/intel/i945/gma.ramstage.o
    src/northbridge/intel/i945/gma.c: In function 'intel_gma_init':
    src/northbridge/intel/i945/gma.c:398:2: error: implicit declaration of function 'vga_textmode_init' [-Werror=implicit-function-declaration]

Selecting the Kconfig variable VGA makes the declaration of the
function `vga_textmode_init()` to be included by the preprocessor.

[1] http://review.coreboot.org/6723

Change-Id: Iecbb2898193078b8738425cea13cb7e6da508cab
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/6947
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
Tested-by: build bot (Jenkins)