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

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 <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>