fix ninja in git bash in windows 10

uname -a in "git bash" in Windows 10 returns "MINGW64*" instead of
"MINGW32*"

Change-Id: I54b1adb1505eb7ebc92fb396b1d07f1fa0795cec
diff --git a/ninja b/ninja
index 33ef701..3ed4f5d 100755
--- a/ninja
+++ b/ninja
@@ -37,6 +37,7 @@
   Darwin)    exec "${THIS_DIR}/ninja-mac" "$@";;
   CYGWIN*)   exec cmd.exe /c $(cygpath -t windows $0-win).exe "$@";;
   MINGW32*)  cmd.exe //c $0-win.exe "$@";;
+  MINGW64*)  cmd.exe //c $0-win.exe "$@";;
   *)         echo "Unsupported OS ${OS}"
              print_help
              exit 1;;