perf tools: Add the include/perf/ directory to .gitignore

perf tools: Add the include/perf/ directory to .gitignore

Commit 3af1dfdd51e06697 ("perf build: Move perf_dlfilters.h in the
source tree") moved perf_dlfilters.h to the include/perf/ directory
while include/perf is ignored because it has 'perf' in the name.  Newly
created files in the include/perf/ directory will be ignored.

Testing:

Before:

  $ touch tools/perf/include/perf/junk
  $ git status | grep junk
  $ git check-ignore -v tools/perf/include/perf/junk
  tools/perf/.gitignore:6:perf    tools/perf/include/perf/junk

After:

  $ git status | grep junk
  tools/perf/include/perf/junk
  $ git check-ignore -v tools/perf/include/perf/junk

Add !include/perf/ to perf's .gitignore file.

Fixes: 3af1dfdd51e06697 ("perf build: Move perf_dlfilters.h in the source tree")
Signed-off-by: Donglin Peng <dolinux.peng@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221103092704.173391-1-dolinux.peng@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>