cogl-private.h: include cogl-context.h
cogl-private.h: include cogl-context.h

cogl-private.h contains prototypes depending on CoglContext so
explicitly include cogl-context.h

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

Distrotech/cogl,Distrotech/cogl,djdeath/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,gcampax/cogl,Distrotech/cogl,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl-android,djdeath/cogl