mingw.h: add dummy functions for sigset_t operations

Windows does not have POSIX-like signals, and so we ignore all
operations on the non-existent signal mask machinery.

Do not turn sigemptyset into a function, but leave it a macro that
erases the code in the argument because it is used to set sa_mask
of a struct sigaction, but our dummy in mingw.h does not have that
member.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>