This fixes some minor errors and warnings that were preventing Cogl
building with mingw32:
• cogl-framebuffer-gl.c was not including cogl-texture-private.h.
Presumably something else ends up including that when building for
GLX.
• The WGL winsys was not including cogl-error-private.h
• A call to strsplit in the WGL winsys was wrong.
• For some reason the test-wrap-rectangle-textures test was trying to
include the GDKPixbuf header.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 5380343399f834d9f96ca3b137d49c9c2193900a)