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: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>