build-sys: add slirp.wrap

This allows to build with --enable-slirp / -D slirp=enabled, even when
libslirp is not installed on the system.  "meson subprojects download"
will pull it from git in that case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230302131848.1527460-4-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.gitignore

@@ -20,3 +20,5 @@ GTAGS
 *.swp
 *.patch
 *.gcov
+
+/subprojects/slirp

subprojects/slirp.wrap

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