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

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>