perf tools: Update copy of libbpf's hashmap.c

To pick the changes in:

  a3e7e6b1 ("libbpf: Remove HASHMAP_INIT static initialization helper")

That don't entail any changes in tools/perf.

This addresses this perf build warning:

  Warning: Kernel ABI header differences:
    diff -u tools/perf/util/hashmap.h tools/lib/bpf/hashmap.h

Not a kernel ABI, its just that this uses the mechanism in place for
checking kernel ABI files drift.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>