configure: accept --enable-slirp

Meson understands -Dslirp=enabled, so there is no reason not to
accept the configure option as well.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1082,6 +1082,8 @@ for opt do
   ;;
   --disable-slirp) slirp="disabled"
   ;;
+  --enable-slirp) slirp="enabled"
+  ;;
   --enable-slirp=git) slirp="internal"
   ;;
   --enable-slirp=system) slirp="system"