Include strings.h for strcasecmp(3).

From Tim Rice

plugins/sudoers/pwutil.c

@@ -32,6 +32,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifdef HAVE_STRINGS_H
+# include <strings.h>		/* strcasecmp */
+#endif
 #ifdef HAVE_SETAUTHDB
 # include <usersec.h>
 #endif /* HAVE_SETAUTHDB */