win32: suppress warnings

* win32/file.c (home_dir, code_page_i): parenthesize to suppress
  warnings.

* win32/win32.c (is_command_com, join_argv, w32_cmdvector, kill),
  (constat_attr_color_reverse, constat_attr): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48971 b2dd03c8-39d4-4d8f-98ff-823fe69b080e