perf build: Add config/feature-checks/*.output to the .gitignore file

They are all auto-generated files during the perf building.

Before this patch:

 $ git status
 Untracked files:
   (use "git add <file>..." to include in what will be committed)

	config/feature-checks/test-all.make.output
	config/feature-checks/test-backtrace.make.output
	config/feature-checks/test-bionic.make.output
	config/feature-checks/test-dwarf.make.output
	config/feature-checks/test-fortify-source.make.output
	config/feature-checks/test-glibc.make.output
	config/feature-checks/test-gtk2-infobar.make.output
	config/feature-checks/test-gtk2.make.output
	config/feature-checks/test-libaudit.make.output
	config/feature-checks/test-libbabeltrace.make.output
	config/feature-checks/test-libbfd.make.output
	config/feature-checks/test-libdw-dwarf-unwind.make.output
	config/feature-checks/test-libelf-getphdrnum.make.output
	config/feature-checks/test-libelf-mmap.make.output
	config/feature-checks/test-libelf.make.output
	config/feature-checks/test-libnuma.make.output
	config/feature-checks/test-libperl.make.output
	config/feature-checks/test-libpython-version.make.output
	config/feature-checks/test-libpython.make.output
	config/feature-checks/test-libslang.make.output
	config/feature-checks/test-libunwind.make.output
	config/feature-checks/test-pthread-attr-setaffinity-np.make.output
	config/feature-checks/test-stackprotector-all.make.output
	config/feature-checks/test-sync-compare-and-swap.make.output
	config/feature-checks/test-timerfd.make.output
	config/feature-checks/test-zlib.make.output

After this patch:

 $ git status
 nothing to commit, working directory clean
Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1426821638-11227-3-git-send-email-yunlong.song@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/config/feature-checks/.gitignore

 *.d
 *.bin
+*.output