fix unused variable warnings in new nextafter/nexttoward code

apparently initializing a variable is not "using" it but assigning to
it is "using" it. i don't really like this fix, but it's better than
trying to make a bigger cleanup just before a release, and it should
work fine (tested against nsz's math tests).

src/math/nexttowardf.c

@@ -31,7 +31,8 @@ float nexttowardf(float x, long double y)
 		return x + x;
 	/* raise underflow if ux.value is subnormal or zero */
 	if (e == 0) {
-		volatile float z = x*x + ux.value*ux.value;
+		volatile float z;
+		z = x*x + ux.value*ux.value;
 	}
 	return ux.value;
 }