When linking directly to libX11 as a hard dependency, we assign the
addresses of functions like XDestroyImage to function pointers like
X11_XDestroyImage. However, by default Xutils.h doesn't declare
XDestroyImage as a function: it only provides a macro which looks into
the XImage struct and calls a function pointer directly, similar to
a virtual method.
SDL_x11framebuffer.c was already relying on being able to call the macro
without explicitly linking to libX11, so do the same here.
Fixes: d14cbd7b "Introduce X11 toolkit and make message dialogs use it"
Signed-off-by: Simon McVittie <smcv@collabora.com>
