Include string.h unconditionally and only use strings.h for strn?casecmp()

In the pre-POSIX days BSD had strings.h, not string.h.
Now strings.h is only used for non-ANSI string functions.

plugins/sudoers/ldap_util.c

@@ -29,12 +29,7 @@
 #include <sys/socket.h>
 #include <stdio.h>
 #include <stdlib.h>
-#ifdef HAVE_STRING_H
-# include <string.h>
-#endif /* HAVE_STRING_H */
-#ifdef HAVE_STRINGS_H
-# include <strings.h>
-#endif /* HAVE_STRINGS_H */
+#include <string.h>
 #include <ctype.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>