fix variable aliasing in x509storeissuer

threadcount is defined globally and in main, which is fine, but doing so
causes assignments to use the local scope rather than the global, which
results in div-by-zero in the thread function when we divide NUM_CALLS
by threadcount

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from #182)

perf/x509storeissuer.c

@@ -65,7 +65,7 @@ static void do_x509storeissuer(size_t num)
 
 int main(int argc, char *argv[])
 {
-    int threadcount, i;
+    int i;
     OSSL_TIME duration, av;
     uint64_t us;
     double avcalltime;