The cogl-texture-private.h needs to be included as
_cogl_texture_needs_premult_conversion, so that we can avoid implicit
declaration warnings of that symbol (aka C4013 on MSVC).
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 4afe9dc1fea646e2a9576f9a0dbd1ffafa40485b)