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_compat.h

@@ -24,6 +24,7 @@
 #ifndef SUDO_COMPAT_H
 #define SUDO_COMPAT_H
 
+#include <sys/types.h>	/* for gid_t, mode_t, size_t, ssize_t, uid_t */
 #include <stdio.h>
 #if !defined(HAVE_VSNPRINTF) || !defined(HAVE_VASPRINTF) || \
     !defined(HAVE_VSYSLOG) || defined(PREFER_PORTABLE_SNPRINTF)