Fix compiler warning in smount example program from sharedsubtree.txt

If one compiles the example smount program, found in
Documentation/sharedsubtree.txt, with -Wall then there's a small
compiler warning rearing its ugly head :

 smount.c: In function 'main':
 smount.c:45: warning: implicit declaration of function 'strcmp'

Easily fixed by just including string.h
Signed-off-by: NJesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: NAdrian Bunk <bunk@kernel.org>

Documentation/sharedsubtree.txt

@@ -153,6 +153,7 @@ replicas continue to be exactly same.
 	#include <stdio.h>
 	#include <stdlib.h>
 	#include <unistd.h>
+	#include <string.h>
 	#include <sys/mount.h>
 	#include <sys/fsuid.h>