net: Unbreak userspace usage of linux/mroute.h

Nothing in linux/pim.h should be exported to userspace.

This should fix the XORP build failure reported by
Jose Calhariz, the debain package maintainer.

Nothing originally in linux/mroute.h was exported to userspace
ever, but some of this stuff started to be when it was moved into
this new linux/pim.h, and that was wrong.  If we didn't provide these
definitions for 10 years we can reasonably expect that applications
defined this stuff locally or used GLIBC headers providing the
protocol definitions.  And as such the only result of this can
be conflict and userland build breakage.

Signed-off-by: David S. Miller <davem@davemloft.net>