Jiri reported that 'make .o' stopped working:

  > [jolsa@krava perf]$ make -f Makefile perf.o
  > cc    -c -o perf.o perf.c
  > In file included from builtin.h:4:0,
  >                  from perf.c:9:
  > util/util.h:74:24: fatal error: lk/debugfs.h: No such file or directory
  > compilation terminated.
  > make: *** [perf.o] Error 1

This is due to GNU make having built-in rules for popular targets such
as *.o. Clear them out so that all targets as passed through to Makefile.perf.
Reported-by: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/n/tip-5wkuvmlaaxtfgepKcvRij8sh@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>