gpg: Fix build on Windows.

WIN32_LEAN_AND_MEAN is required to avoid definitions of grp1, grp2,
and grp3 in dlgs.h, which is included by windows.h.

• Fixes-commit: fd595c9d3642dba437fbe0f6e25d7aaaae095f94
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>