slirp: bump submodule past 4.7 release

Version 4.7 of slirp provides a new timer API that works better with CFI,
together with several other improvements:

* Allow disabling the internal DHCP server !22
* Support Unix sockets in hostfwd !103
* IPv6 DNS proxying support !110
* bootp: add support for UEFI HTTP boot !111

and bugfixes.

The submodule update also includes 2 commits to fix warnings in the
Win32 build.

Reviewed-by: Marc-André Lureau <malureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>