pimd: relegate pim_igmp_join.c into a header file

pimd: relegate pim_igmp_join.c into a header file

pim_igmp_join.c only exists to make some portability hacks available to
test_igmpv3_join.  The function only has 1 call site in each pimd and
the test tool, so it's nicely served as a simple static function in a
header file.

This removes a MTYPE related compiler/linker issue from referencing
lib/if.h in a binary that doesn't link libzebra, as test_igmpv3_join is
now fully independent of lib/.

(Fix by Christian Franke: remove stray leftover ifindex_t)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>