tools/bpftool: ignore build products

For untracked things of tools/bpf, add this.
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/.gitignore

+FEATURE-DUMP.bpf
+bpf_asm
+bpf_dbg
+bpf_exp.yacc.*
+bpf_jit_disasm

tools/bpf/bpftool/.gitignore

 *.d
 bpftool
+bpftool*.8
+bpf-helpers.*
 FEATURE-DUMP.bpftool