perf tools: Fix build breakage

The patch ecafda60

 introduced a problem where all object files would be
always rebuilt, fix it by using:

http://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Bernd Petrovitsch <bernd@sysprog.at>
Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>