perf tools: Fix dynamic libbpf link

perf tools: Fix dynamic libbpf link

Justin reported broken build with LIBBPF_DYNAMIC=1.

When linking libbpf dynamically we need to use perf's
hashmap object, because it's not exported in libbpf.so
(only in libbpf.a).

Following build is now passing:

  $ make LIBBPF_DYNAMIC=1
    BUILD:   Doing 'make -j8' parallel build
    ...
  $ ldd perf | grep libbpf
        libbpf.so.0 => /lib64/libbpf.so.0 (0x00007fa7630db000)

Fixes: eee19501926d ("perf tools: Grab a copy of libbpf's hashmap")
Reported-by: Justin M. Forbes <jforbes@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210508205020.617984-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>