Add gnuwin32\bin directory to PATH on Windows

This is necessary for finding gperf, bison, yacc, etc. on Windows.
Works only if the gnuwin32 directory is on the same level as the
qtwebengine root directory, which is the case with a qt checkout.

This is a port of 09e53e79

 which did the same for the gyp build.

Change-Id: Icd75c44605ba07269f721cd3b9877463c43f8beb
Reviewed-by: Michal Klocek <michal.klocek@qt.io>