Suppress -Wunused-result warning for strtol().

I'm not sure which bozo thought it's a problem to use strtol() only
for its endptr result, but silence the warning using same method
used elsewhere.

Report: <f845d3a6-5328-3e2a-924f-f8e91aa2b6d2@2ndquadrant.com>

src/backend/utils/adt/dbsize.c

@@ -759,13 +759,15 @@ pg_size_bytes(PG_FUNCTION_ARGS)
 	/* Part (4): optional exponent */
 	if (*endptr == 'e' || *endptr == 'E')
 	{
+		long		exponent;
 		char	   *cp;
 
 		/*
 		 * Note we might one day support EB units, so if what follows 'E'
 		 * isn't a number, just treat it all as a unit to be parsed.
 		 */
-		(void) strtol(endptr + 1, &cp, 10);
+		exponent = strtol(endptr + 1, &cp, 10);
+		(void) exponent;		/* Silence -Wunused-result warnings */
 		if (cp > endptr + 1)
 			endptr = cp;
 	}