perf: Fix reproducibility issue

perf: Fix reproducibility issue

Perf's build process generates two copies of its internal headers,
one at tools/lib/perf/include/internal and one at libperf/include/internal
with files like xyarray.h. Although the files are identical, the binaries including
them would see them at different paths and have different debuginfo data.

To avoid this, build libperf.a first, which will ensure the libperf
headers directory exists. This is used in preference to the other by
the build process and results in consistent binaries.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>