drmmode_display: fix missing include of dpmsconst.h

Don't rely on some Xserver SDK header is silently doing it
by accident.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

Author: metux

src/drmmode_display.h

@@ -32,6 +32,8 @@
 #include "libudev.h"
 #endif
 
+#include <X11/extensions/dpmsconst.h>
+
 #include "amdgpu_drm_queue.h"
 #include "amdgpu_probe.h"
 #include "amdgpu.h"