Windows: git-shell can be compiled again

Windows: git-shell can be compiled again

The reason that git-shell was excluded from the Windows build was that
our compatibility layer needed stuff that was removed when we tried to
link less of the git library into git-shell. Since 4cfc24a (shell: do
not play duplicated definition games to shrink the executable,
2008-08-19) the complete library is linked again, so we can build
git-shell on Windows as well. (This fixes 'make install', which depends
on that git-shell is always built.)

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>