slirp: update wrap to latest master

It is recommended to use SSIZE_T for ssize_t on win32, but the commit
that is being used for slirp.wrap uses int.  Update to include the fix
as well as the other bugfix commit "ip: Enforce strict aliasing".

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

subprojects/slirp.wrap

 [wrap-git]
 url = https://gitlab.freedesktop.org/slirp/libslirp
-revision = 15c52d697529eb3e78c5d8aa324d61715bce33b6
+revision = 26be815b86e8d49add8c9a8b320239b9594ff03d
 
 [provide]
 slirp = libslirp_dep