Use atomics for allocation counter

I was getting cases where the CPU cache was causing issues with the
allocation counter, for the longest time I thought I was doing something
wrong, but when the allocation counter went below 0, I realized it was
because I didn't use atomics for incrementing/decrementing the
allocation counter variable.  The allocation counter now always should
have the correct value.

obs/obs-app.cpp

@@ -233,7 +233,6 @@ int main(int argc, char *argv[])
 		blog(LOG_ERROR, "%s", error);
 	}
 
-	blog(LOG_INFO, "Number of memory leaks: %llu",
-			(unsigned long long int)bnum_allocs());
+	blog(LOG_INFO, "Number of memory leaks: %ld", bnum_allocs());
 	return ret;
 }