cogl/cogl-offscreen.h: Fix deprecation macro placement
cogl/cogl-offscreen.h: Fix deprecation macro placement

The deprecation macros need to be before the function prototype on
Visual Studio, and is also accepted by GCC.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl