samples/bpf: Update .gitignore

Update samples/bpf/.gitignore to ignore files generated when building
the samples. Add:

  - vmlinux.h
  - the generated skeleton files (*.skel.h)
  - the samples/bpf/libbpf/ and .../bpftool/ directories, in preparation
    of a future commit which introduces a local output directory for
    building libbpf and bpftool.
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007194438.34443-9-quentin@isovalent.com

samples/bpf/.gitignore

@@ -57,3 +57,7 @@ testfile.img
 hbm_out.log
 iperf.*
 *.out
+*.skel.h
+/vmlinux.h
+/bpftool/
+/libbpf/