Fix Qt's moc path on Windows

Backslash from mocCmdBase is processed as an escape sequence in
generated gyp file. E.g. Qt5\bin\moc.exe turns to
Qt5[BS]in\moc.exe.

Change-Id: I09a04b0e168e1139720c44307f192f7a9368ad43
Reviewed-by: Michael Bruning <michael.bruning@digia.com>