[slirp] Disable warnings for uncleanly deprecated libslirp functions

[slirp] Disable warnings for uncleanly deprecated libslirp functions

libslirp introduced a new API for constructing polling lists, to
accommodate Windows platforms where a handle descriptor may be too
large for an int.

Older versions of libslirp do not have the new API calls, and the
older API calls were immediately marked as deprecated, with no
overlap.  We would therefore need to use #ifdef and always have some
code that is deliberately not compiled, depending on the version of
libslirp that we find on the user's system.  This is highly
undesirable.

Work around this by disabling the deprecation warning (which is what
libslirp itself does for the portions of its code that necessarily
touch the deprecated functions).

Signed-off-by: Michael Brown <mcb30@ipxe.org>