Ensure that QT5_BASE_DIR is known by QtShark.pro

Otherwise the call to winqtdeploy will fail if it is not already in the PATH

Change-Id: I74db604a6fd45204bda8d6ac9d9c98c0c12598ec
Reviewed-on: https://code.wireshark.org/review/3218
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>