Add an empty libperf.a under tools/perf/lib and link it with perf.

It can also be built separately with:

  $ cd tools/perf/lib && make
    CC       core.o
    LD       libperf-in.o
    AR       libperf.a
    LINK     libperf.so

Committer testing:

  $ make O=/tmp/build/perf -C tools/perf/lib/
  make: Entering directory '/home/acme/git/perf/tools/perf/lib'
    LINK     /tmp/build/perf/libperf.so
  make: Leaving directory '/home/acme/git/perf/tools/perf/lib'
  $ ls -la /tmp/build/perf/libperf.so
  -rwxrwxr-x. 1 acme acme 16232 Jul 22 15:30 /tmp/build/perf/libperf.so
  $ file /tmp/build/perf/libperf.so
  /tmp/build/perf/libperf.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=7a51d227d871b381ddb686dcf94145c4dd908221, not stripped
  $ git status tools/perf
  On branch perf/core
  nothing to commit, working tree clean
  $
  $ ls -lart tools/perf/lib/
  total 16
  drwxrwxr-x. 16 acme acme 4096 Jul 22 15:29 ..
  -rw-rw-r--.  1 acme acme 1633 Jul 22 15:29 Makefile
  -rw-rw-r--.  1 acme acme    0 Jul 22 15:29 core.c
  -rw-rw-r--.  1 acme acme   20 Jul 22 15:29 Build
  drwxrwxr-x.  2 acme acme 4096 Jul 22 15:29 .
  $

Committer notes:

Need to add -I$(srctree)/tools/arch/$(ARCH)/include/uapi
-I$(srctree)/tools/include/uapi to tools/perf/lib/Makefile's INCLUDE
variable to pick up the latest versions of kernel headers, even in older
systems, this is in line with what is in tools/lib/bpf/Makefile.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-24-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>