include strings.h for strcasecmp

while this is not the only place strcasecmp is used without an
explicit inclusion of the header, it's the only place gcc complains
on my environment.  for other places, probably the header is included
indirectly via other headers, i guess.

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>

lib/smap.c

@@ -15,6 +15,8 @@
 #include <config.h>
 #include "smap.h"
 
+#include <strings.h>
+
 #include "hash.h"
 #include "json.h"