sd-hwdb: include sys/stat.h in hwdb-internal.h

Include this header to fix errors when including hwdb-internal.h:
  ../src/libsystemd/sd-hwdb/hwdb-internal.h:16:21: error: field ‘st’ has incomplete type
     16 |         struct stat st;

Author: enr0n

src/libsystemd/sd-hwdb/hwdb-internal.h

@@ -2,6 +2,7 @@
 #pragma once
 
 #include <stdint.h>
+#include <sys/stat.h>
 
 #include "def.h"
 #include "hashmap.h"