obj_refer_only_sharables_p_i: need derefernce

Clang says "warning: variable 'pcnt' set but not used" here.  In fact it
doesn't. The intention is clear that we want to increment cnt, not pcnt.
Adding a * mark solves everything.

ractor.c

@@ -2685,7 +2685,7 @@ obj_refer_only_shareables_p_i(VALUE obj, void *ptr)
     int *pcnt = (int *)ptr;
 
     if (!rb_ractor_shareable_p(obj)) {
-        pcnt++;
+        *pcnt++;
     }
 }