Fix a small build issue.

Strcmp in SampleApplication.cpp is a cstring function. So some
compilers (at least the default gcc compiler in my Ubuntu 17.04)
would think that strcmp is not declared and lead to build failure,
even we have include <string> in header file.

Now that we use std::string variable in SampleApplication.h, it is
better to include <string> in that header file as what it is.
Then add <string.h> to include cstring functions in .cpp file to
fix the build issue.

BUG=angleproject:2180

Change-Id: Id97a6956480637c1218314c04d9b331031c7b911
Reviewed-on: https://chromium-review.googlesource.com/711396
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

samples/sample_util/SampleApplication.cpp

@@ -11,6 +11,7 @@
 #include "angle_gl.h"
 
 #include <iostream>
+#include <string.h>
 
 namespace
 {