bpftool: Add _bpftool and profiler.skel.h to .gitignore

These files are generated, so ignore them.
Signed-off-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: NQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200312182332.3953408-4-songliubraving@fb.com

tools/bpf/bpftool/.gitignore

 *.d
+/_bpftool
 /bpftool
 bpftool*.8
 bpf-helpers.*
 FEATURE-DUMP.bpftool
 feature
 libbpf
+profiler.skel.h