Port emacsclient to FreeBSD 11 etc.

* lib-src/emacsclient.c: Include <string.h>.  This is needed on
platforms like FreeBSD 11 that use code involving strlen etc., and
because <config.h> no longer includes string.h.

lib-src/emacsclient.c

@@ -75,6 +75,7 @@ char *w32_getenv (const char *);
 #include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <getopt.h>
 #include <unistd.h>