tools/bpf: add missing strings.h include

Few files in libbpf are using bzero() function (defined in strings.h header), but
don't include corresponding header. When libbpf is added as a dependency to pahole,
this undeterministically causes warnings on some machines:

bpf.c:225:2: warning: implicit declaration of function 'bzero' [-Wimplicit-function-declaration]
  bzero(&attr, sizeof(attr));
    ^~~~~

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>