gtkmountoperation-x11: unbreak compilation on OpenBSD.

Add missing header to prevent undeclared definitions.
Bug #659565

gtk/gtkmountoperation-x11.c

@@ -42,6 +42,13 @@
 #include <signal.h>
 #include <errno.h>
 
+#if defined(__OpenBSD__)
+#include <sys/param.h>
+#include <kvm.h>
+#include <fcntl.h>
+#include <sys/sysctl.h>
+#endif
+
 #include "gtkmountoperationprivate.h"
 
 #include "gtkalias.h"