lib: add gcc 4.x workaround for frr_interface_info

lib: add gcc 4.x workaround for frr_interface_info

gcc 4.x does not properly support structs with variable length array
members.  Specifically, for global variables, it completely ignores the
array, coming up with a size much smaller than what is correct.  This is
broken for both sizeof() as well as ELF object size.

This breaks for frr_interface_info since this variable is in some cases
copy relocated by the linker.  (The linker does this to make the address
of the variable a "constant" for the main program.)  This copying uses
the ELF object size, thereby copying only the non-array part of the
struct.

Breakage ensues...

(This fix is a bit ugly, but it's limited to very old gcc, and it's
better than changing the array to "nodes[1000]" and wasting memory...)

Fixes: #4563
Fixes: #5074
Signed-off-by: David Lamparter <equinox@diac24.net>