Add dependent system includes to make sudo_*.h more standalone.

In the past we've relied on the various .c files to include the
system headers that define types that the sudo_*.h headers require.
This is fragile and can cause issues when includes get re-ordered.

include/sudo_util.h

@@ -19,6 +19,7 @@
 #ifndef SUDO_UTIL_H
 #define SUDO_UTIL_H
 
+#include <sys/types.h>	/* for dev_t, mode_t, gid_t, size_t, ssize_t, uid_t */
 #ifdef HAVE_STDBOOL_H
 # include <stdbool.h>
 #else