qt5_tool: fix for perl v5.8.8

The perl version which comes with msysgit is v5.8.8. There, the
script fails with

   Undefined subroutine &Cwd::realpath called at
       qtrepotools/bin/qt5_tool line 600.

In this perl version Cwd's realpath apparently needs to be
explicitly exported.

Change-Id: I467c5879e3b67e14652054fc9df1ce9fd547aa14
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>