libbpf: Fix ELF symbol visibility update logic

Fix silly bug in updating ELF symbol's visibility.

Fixes: a4634922 ("libbpf: Add linker extern resolution support for functions and global variables")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210507054119.270888-6-andrii@kernel.org