perf tools: Ignore Documentation dependency file

perf tools: Ignore Documentation dependency file

When building directly on the checked out repository the build process
produces a file that should be ignored, so add it to .gitignore.

Fixes: 3dc5579b27613847 ("perf doc: Fix doc.dep")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210910232249.739661-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>