libbpf: Silence uninitialized warning/error in btf_dump_dump_type_data

When compiling libbpf with gcc 4.8.5, we see:

  CC       staticobjs/btf_dump.o
btf_dump.c: In function ‘btf_dump_dump_type_data.isra.24’:
btf_dump.c:2296:5: error: ‘err’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (err < 0)
     ^
cc1: all warnings being treated as errors
make: *** [staticobjs/btf_dump.o] Error 1

While gcc 4.8.5 is too old to build the upstream kernel, it's possible it
could be used to build standalone libbpf which suffers from the same problem.
Silence the error by initializing 'err' to 0.  The warning/error seems to be
a false positive since err is set early in the function.  Regardless we
shouldn't prevent libbpf from building for this.

Fixes: 920d16af ("libbpf: BTF dumper support for typed data")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1638180040-8037-1-git-send-email-alan.maguire@oracle.com