Fix ninja wrapper to work on MINGW64

Small fix that makes it possible to use ninja and depot-tools using
the mingw64 chroot bundled in git 2.7.0
With this change I can build succesfully on Windows and still have
a civilized shell.

BUG=
TBR=maruel@chromium.org

Review URL: https://codereview.chromium.org/1638303006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298454 0039d316-1c4b-4281-b951-d872f2087c98

ninja

@@ -36,7 +36,7 @@ case "$OS" in
     ;;
   Darwin)    exec "${THIS_DIR}/ninja-mac" "$@";;
   CYGWIN*)   exec cmd.exe /c $(cygpath -t windows $0).exe "$@";;
-  MINGW32*)  cmd.exe //c $0.exe "$@";;
+  MINGW*)    cmd.exe //c $0.exe "$@";;
   *)         echo "Unsupported OS ${OS}"
              print_help
              exit 1;;