Fix ninja wrapper to work on MINGW64
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: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@298454 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools