bpf: Compile out btf_parse_module() if module BTF is not enabled

Make sure btf_parse_module() is compiled out if module BTFs are not enabled.

Fixes: 36e68442

 ("bpf: Load and verify kernel module BTFs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201111040645.903494-1-andrii@kernel.org