tools lib traceevent: Fix off-by-one bug in pevent_strerror()

As pevent_errno is defined using PEVENT_ERRORS which uses _PE macro
magic, the first errno is bigger than __PEVENT_ERRNO_START by 1. So we
need to subtract the 1 also when calculating the index of the error
strings.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1345707420-21767-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>