cext: move util.h to cext tree

Since util.h isn't useful in plain C module, it should be placed in CPython
extension directory.

setup.py

@@ -591,7 +591,7 b" packages = ['mercurial',"
 
 common_depends = ['mercurial/bitmanipulation.h',
                   'mercurial/compat.h',
-                  'mercurial/util.h']
+                  'mercurial/cext/util.h']
 common_include_dirs = ['mercurial']
 
 osutil_cflags = []