configure: Document --disable-slirp option in --help

The --disable-slirp option was undocumented; add it to configure's
--help output.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/configure b/configure
index bc54293..a318d37 100755
--- a/configure
+++ b/configure
@@ -966,6 +966,7 @@
 echo "  --enable-check-utests    enable check unit-tests"
 echo "  --disable-bluez          disable bluez stack connectivity"
 echo "  --enable-bluez           enable bluez stack connectivity"
+echo "  --disable-slirp          disable SLIRP userspace network connectivity"
 echo "  --disable-kvm            disable KVM acceleration support"
 echo "  --enable-kvm             enable KVM acceleration support"
 echo "  --disable-nptl           disable usermode NPTL support"