pim_mroute.h has a different version of code than linux/mroute.h provides

pim_mroute.h has a different version of code than linux/mroute.h provides

linux/mroutes.h and pim_mroute.h both have copies of the same structures.
This is causing failures in setsockopt(..., MRT_ADD_MFC,...) because
of data structure incompatibilities between the kernel and what
pim_mroute.h was providing.  Modify the code to check for mroute.h
and include it if necessary.  I did not modify the non linux/mroute.h
path because I do not have other systems to test on easily.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>