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 <rpjday@crashcourse.ca>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>