Add python-spwd depenency to rest server

Needed for pending rest server authentication support.
diff --git a/common/recipes-phosphor/dbus/obmc-rest.bb b/common/recipes-phosphor/dbus/obmc-rest.bb
index 05f2383..cb68953 100644
--- a/common/recipes-phosphor/dbus/obmc-rest.bb
+++ b/common/recipes-phosphor/dbus/obmc-rest.bb
@@ -16,6 +16,7 @@
 	obmc-mapper \
 	python-rocket \
 	python-bottle \
+	python-spwd \
         "
 SRC_URI += " \
         git://github.com/openbmc/phosphor-rest-server \