perf tests: Fix build on older systems where 'signal' is reserved

perf tests: Fix build on older systems where 'signal' is reserved

fixing the following problems, for instance, on RHEL6.7:

    CC       /tmp/build/perf/tests/bp_signal.o
  cc1: warnings being treated as errors
  tests/bp_signal.c: In function ‘__event’:
  tests/bp_signal.c:106: error: declaration of ‘signal’ shadows a global declaration
  /usr/include/signal.h:101: error: shadowed declaration is here
  tests/bp_signal.c: In function ‘bp_event’:
  tests/bp_signal.c:144: error: declaration of ‘signal’ shadows a global declaration
  /usr/include/signal.h:101: error: shadowed declaration is here
  tests/bp_signal.c: In function ‘wp_event’:
  tests/bp_signal.c:149: error: declaration of ‘signal’ shadows a global declaration
  /usr/include/signal.h:101: error: shadowed declaration is here
  mv: cannot stat `/tmp/build/perf/tests/.bp_signal.o.tmp': No such file or directory
  make[3]: *** [/tmp/build/perf/tests/bp_signal.o] Error 1
  make[2]: *** [tests] Error 2
  make[1]: *** [/tmp/build/perf/perf-in.o] Error 2
  make[1]: *** Waiting for unfinished jobs....

Reported-by: Vinson Lee <vlee@freedesktop.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: He Kuang <hekuang@huawei.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: pi3orama@163.com
Fixes: 8fd34e1cce18 ("perf test: Improve bp_signal")
Link: http://lkml.kernel.org/n/tip-wlpx6tik1b0jirlkw64bv400@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>