Bug 530550. Fix non-libxul build breakage caused by 0af6ca3135ca. r=ehsan

Non-libxul builds include pixman.h directly instead of having it included
by pixman-private.h which defines PIXMAN_USE_INTERNAL_API. So we ensure
PIXMAN_USE_INTERNAL_API is defined before including pixman.h

gfx/cairo/libpixman/src/Makefile.in

@@ -175,7 +175,7 @@ ifdef GNU_CC
 # files, so pixman.h needs to be included before
 # "#pragma GCC visibility -push(hidden)".
 ifdef WRAP_SYSTEM_INCLUDES
-MY_VISIBILITY_FLAGS := -include pixman.h $(VISIBILITY_FLAGS)
+MY_VISIBILITY_FLAGS := -DPIXMAN_USE_INTERNAL_API -include pixman.h $(VISIBILITY_FLAGS)
 COMPILE_CFLAGS += $(MY_VISIBILITY_FLAGS)
 endif # WRAP_SYSTEM_INCLUDES
 endif # GNU_CC