filters: fix incorrect #if for vf_gpu

This didn't match what is in wscript_build.py. Also, it should work on
non-X11 platforms... probably. (The condition is convoluted and almost
nonsensical, but the offscreen context creation needs to be cleaned up
anyway as soon as other backends, e.g. for win32, are added.)

filters/user_filters.c

@@ -85,7 +85,7 @@ const struct mp_user_filter_entry *vf_list[] = {
 #if HAVE_D3D_HWACCEL
     &vf_d3d11vpp,
 #endif
-#if HAVE_EGL_X11
+#if HAVE_EGL_HELPERS && HAVE_GL && HAVE_EGL15
     &vf_gpu,
 #endif
 };