sh: Fix up HAVE_PERF_COUNTERS typo.
That's HAVE_PERF_COUNTERS not HAVE_PERF_COUNTER. This was right initially but I seem to have screwed it up while re-typing it out by hand on another machine when I checked it in. Hmph. Reported-by: Robert P. J. Day <email@example.com> Signed-off-by: Paul Mundt <firstname.lastname@example.org>