pull in string prototypes

--- gdk/gdkinputcommon.h
+++ gdk/gdkinputcommon.h
@@ -24,6 +24,8 @@
  * GTK+ at ftp://ftp.gtk.org/pub/gtk/. 
  */
 
+#include <string.h>
+
 #if defined(XINPUT_GXI) || defined(XINPUT_XFREE)
 
 /* Forward declarations */