[core] fix "--disable-libyuv"

[core] fix "--disable-libyuv"

Recent changes made it impossible to compile freeswitch without libyuv
support.

src/switch_core_video.c: In function 'switch_img_read_from_file':
src/switch_core_video.c:3139:4: error: implicit declaration of function 'RAWToI420' [-Werror=implicit-function-declaration]
RAWToI420(data, width * 3,
^
src/switch_core_video.c:3148:4: error: implicit declaration of function 'ABGRToARGB' [-Werror=implicit-function-declaration]
ABGRToARGB(data, width * 4, img->planes[SWITCH_PLANE_PACKED], img->stride[SWITCH_PLANE_PACKED], width, height);
^

Fix this my adding/moving the checks for "SWITCH_HAVE_YUV".

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>