openvswitch: make dependencies conditional for openvswitch-python subpackage

Apparently, even if you don't select the `openvswitch-python` package,
Python still gets built (for the target build).

But, if the python dependencies are conditional on the
`PACKAGE_openvswitch-python` symbol then they aren't build.

This should improve build times, if you only want to build the
`openvswitch` package.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

net/openvswitch/Makefile

@@ -67,7 +67,7 @@ endef
 define Package/openvswitch-python
   $(call Package/openvswitch/Default)
   TITLE:=Open vSwitch Python Support
-  DEPENDS:=+openvswitch +python +python-six
+  DEPENDS:=+openvswitch +PACKAGE_openvswitch-python:python +PACKAGE_openvswitch-python:python-six
 endef
 
 define Package/openvswitch-python/description