Fix return type of gettemperature

I get this error when trying to build my NixOS system with the nixos-25.05 channel:

dwmstatus.c: In function 'gettemperature':
dwmstatus.c:182:24: error: returning 'char *' from a function with return type 'int' makes integer from pointer without a cast []
  182 |                 return smprintf("");
      |                        ^~~~~~~~~~~~

dwmstatus.c

@@ -179,9 +179,8 @@ gettemperature(char *base, char *sensor)
 
 	co = readfile(base, sensor);
 	if (co == NULL)
-		return smprintf("");
-  return atof(co) / 1000;
-	// return smprintf("%02.0f°C", atof(co) / 1000);
+		return 0;
+	return atof(co) / 1000;
 }
 
 char *