perf tools: Fix time function double declaration with glibc

It's enough to include the local "debug.h" file to trigger it.

man time reveals this is already declared in glibc:

time - get time in seconds
-> rename the variable.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: arjan@infradead.org
LPU-Reference: <1295620209-13859-2-git-send-email-trenn@suse.de>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>