Add 'tools/perf/libbpf/' to ignored files

Commit 6b491a86 ("perf build: Install libbpf headers locally when
building") installed copies of the libbpf headers into the build tree,
causing unnecessary noise from 'git status' after a perf tools build.

Add the 'libbpf/' subdirectory to the .gitignore file to silence it all
again.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/perf/.gitignore

@@ -37,6 +37,7 @@ trace/beauty/generated/
 pmu-events/pmu-events.c
 pmu-events/jevents
 feature/
+libbpf/
 fixdep
 libtraceevent-dynamic-list
 Documentation/doc.dep