Use pid_t data type for target_{pid|tid} vars.

Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <20110205203938.GA15328@hera.kernel.org>
Signed-off-by: Denis Kirjanov <dkirjanov@kernel.org>
[ committer note: those variables are now in struct perf_top, fixed ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>