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/match_command.c

@@ -31,12 +31,7 @@
 #include <sys/stat.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 <unistd.h>
 #ifdef HAVE_GLOB
 # include <glob.h>