perf event: Drop perf_regs.h include, not needed anymore

Since commit c8978997 ("perf tools: Prevent out-of-bounds access
to registers") the util/event.h header doesn't use anything from
util/perf_regs.h, so drop it to untangle the header dependency tree a
bit, speeding up compilation.

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