Correctly include C headers

This patch refactors VIXL to use `extern` block when including C header
that do not have a C++ counterpart.

Change-Id: I203d7d107755dbac3e5f4cf8d2f196f70dea1e07
diff --git a/src/utils-vixl.h b/src/utils-vixl.h
index 06c7551..b27310d 100644
--- a/src/utils-vixl.h
+++ b/src/utils-vixl.h
@@ -27,9 +27,8 @@
 #ifndef VIXL_UTILS_H
 #define VIXL_UTILS_H
 
-#include <string.h>
-
 #include <cmath>
+#include <cstring>
 #include <vector>
 
 #include "compiler-intrinsics-vixl.h"