copy-files.sh: libexec/gnupg/ doesn't exist anymore

Commit 8905816 updated GNUPG and moved the helper executables from
mingw/libexec/gnupg to mingw/bin but didn't update the copy script
in share/WinGit.

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
Merge pull request #96 from snaewe/sn/libexec-gnupg-gone

copy-files.sh: libexec/gnupg/ doesn't exist anymore