git-gui: fix PATH environment for mingw development environment

git-gui: fix PATH environment for mingw development environment

When creating a desktop shortcut from the gui the shortcut directly
starts wish with the git-gui script. In the msysgit development
environment some dll's reside in the mingw/bin directory which causes
that git can not start because libiconv2.dll is not found.

When using such a link the error is even more cryptic stating:
"child killed: unknown signal"

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>