selftests/bpf: satisfy compiler by having explicit return in btf test

[ Upstream commit f4c7e887324f5776eef6e6e47a90e0ac8058a7a8 ]

Some compilers complain about get_pprint_mapv_size() not returning value
in some code paths. Fix with explicit return.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231102033759.2541186-3-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>