pimd: Make frr able to be built by gcc 10

pimd: Make frr able to be built by gcc 10

- Add extern modifier to some declarations in header file and move
qpim_all_pim_routers_addr definition to pimd/pimd.c

`GCC now defaults to -fno-common. As a result, global variable accesses
are more efficient on various targets. In C, global variables with
multiple tentative definitions now result in linker errors.`
Taken from https://gcc.gnu.org/gcc-10/changes.html

Signed-off-by: Tomas Korbar <tkorbar@redhat.com>
(cherry picked from commit 403242d214b6cb919ed08f2149d7bee77a060947)