app: add missing header for XVisualInfo and other X11 declarations

Without this header, the x/mobile build fails on my workstation.
(Compare CL 359695 in x/exp.)

Change-Id: If32fe6bc2945d31fed5cffca5e347970b344e40e
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/362574
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

Author: Bryan C. Mills

app/x11.c

@@ -8,6 +8,7 @@
 #include <EGL/egl.h>
 #include <GLES2/gl2.h>
 #include <X11/Xlib.h>
+#include <X11/Xutil.h>
 #include <stdio.h>
 #include <stdlib.h>