Windows: force-recompile git.res for differing architectures

Windows: force-recompile git.res for differing architectures

When git.rc is compiled into git.res, the result is actually dependent
on the architecture. That is, you cannot simply link a 32-bit git.res
into a 64-bit git.exe.

Therefore, to allow 32-bit and 64-bit builds in the same directory, we
let git.res depend on GIT-PREFIX so that it gets recompiled when
compiling for a different architecture (this works because the exec path
changes based on the architecture: /mingw32/libexec/git-core for 32-bit
and /mingw64/libexec/git-core for 64-bit).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>