Sort and fix `#include` directives.

Includes are sorted into groups, each sorted alphabetically and separated by a
blank line. The groups are, in order, C headers, C++ headers, VIXL (non-backend)
headers, and VIXL backend headers.

Change-Id: I9a71430bffeaf77389a089487ce264db8d1d9ddc
diff --git a/src/code-buffer-vixl.h b/src/code-buffer-vixl.h
index 4223df5..d917f08 100644
--- a/src/code-buffer-vixl.h
+++ b/src/code-buffer-vixl.h
@@ -28,6 +28,7 @@
 #define VIXL_CODE_BUFFER_H
 
 #include <string.h>
+
 #include "globals-vixl.h"
 
 namespace vixl {