libperf build: Always place libperf includes first

When building tools/perf the CFLAGS can contain a directory for the
installed headers.

As the headers may be being installed while building libperf.a this can
cause headers to be partially installed and found in the include path
while building an object file for libperf.a.

The installed header may reference other installed headers that are
missing given the partial nature of the install and then the build fails
with a missing header file.

Avoid this by ensuring the libperf source headers are always first in
the CFLAGS.

Fixes: 3143504918105156 ("libperf: Make libperf.a part of the perf build")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>