Revise include order using clang-format-6.0

Run the `clang-format.bash` script to update our C and C++ code to a new
include order `.clang-format`.  Use `clang-format` version 6.0.

Source/cmFileLockUnix.cxx

@@ -1,13 +1,14 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
-#include "cmFileLock.h"
-
-#include "cmSystemTools.h"
 #include <cerrno> // errno
 #include <cstdio> // SEEK_SET
+
 #include <fcntl.h>
 #include <unistd.h>
 
+#include "cmFileLock.h"
+#include "cmSystemTools.h"
+
 cmFileLock::cmFileLock() = default;
 
 cmFileLockResult cmFileLock::Release()