perf tools: Use python-config --includes rather than --cflags

Builds started failing in Fedora on Python 3.7 with:

    `.gnu.debuglto_.debug_macro' referenced in section
    `.gnu.debuglto_.debug_macro' of
    util/scripting-engines/trace-event-python.o: defined in discarded
    section

In Fedora, Python 3.7 added -flto to the list of --cflags and since it
was only applied to util/scripting-engines/trace-event-python.c and
scripts/python/Perf-Trace-Util/Context.c, linking failed.

It's not the first time the addition of flags has broken builds: commit
c6707fde

 ("perf tools: Fix up build in hardnened environments")
appears to have fixed a similar problem. "python-config --includes"
provides the proper -I flags and doesn't introduce additional CFLAGS.

Signed-off-by: Jeremy Cline <jcline@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180710154612.6285-1-jcline@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>