mingw: skip symlinks pointing to directories

gphoto2 has a infinite loop caused by a symlink, which breaks the
mingw toolchain creation. Instead of adding more checks, skip all
directories.

Change-Id: Iec4fb96da12874dcc4939ca8a8af1f956ef3d164
Reviewed-by: Teemu Holappa <teemu.holappa@theqtcompany.com>