backends/drm: add missing sys/types.h include

This include is needed for the type dev_t used in this file. Issue seen
when compiling with musl libc on Alpine / postmarketOS.

src/backends/drm/drm_backend.h

@@ -16,6 +16,9 @@
 #include <QSocketNotifier>
 #include <QVector>
 
+// system
+#include <sys/types.h>
+
 #include <memory>
 
 struct gbm_bo;