Added include required for XUniqueContext.

src/x11_init.c

diff --git a/src/x11_init.c b/src/x11_init.c
index eba89a7..2781222 100644
--- a/src/x11_init.c
+++ b/src/x11_init.c
@@ -30,6 +30,8 @@
 
 #include "internal.h"
 
+#include <X11/Xresource.h>
+
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>