selftests/bpf: Fix compiler warnings in bpf_testmod for kfuncs

Add -Wmissing-prototypes ignore in bpf_testmod.c, similarly to what we
do in kernel code proper.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/oe-kbuild-all/202304080951.l14IDv3n-lkp@intel.com/
Link: https://lore.kernel.org/bpf/20230412034647.3968143-1-andrii@kernel.org