Fix build warnings from image_loader

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>