Fix build warnings from image_loader

Fix the following build warnings, and the build failures due to the
warning fixes:

  CC     libshared_cairo_la-image-loader.lo
image-loader.c:369:1: warning: no previous prototype for 'load_image'

  CC     x11_backend_la-compositor-x11.lo
compositor-x11.c: In function 'x11_output_set_icon':
compositor-x11.c:396:2: warning: implicit declaration of function 'load_image'
compositor-x11.c:396:8: warning: assignment makes pointer from integer without a cast

  CC     wayland_backend_la-compositor-wayland.lo
compositor-wayland.c: In function 'create_border':
compositor-wayland.c:97:2: warning: implicit declaration of function 'load_image'
compositor-wayland.c:97:8: warning: assignment makes pointer from integer without a cast

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/compositor-x11.c b/src/compositor-x11.c
index b26da09..04a1803 100644
--- a/src/compositor-x11.c
+++ b/src/compositor-x11.c
@@ -48,6 +48,7 @@
 #include "compositor.h"
 #include "gl-renderer.h"
 #include "../shared/config-parser.h"
+#include "../shared/cairo-util.h"
 
 #define DEFAULT_AXIS_STEP_DISTANCE wl_fixed_from_int(10)