tuned signature: fewer warnings in IntelliJ IDEA;

--- a/src/Pure/Build/build_process.scala	Mon Mar 11 23:03:12 2024 +0100
+++ b/src/Pure/Build/build_process.scala	Tue Mar 12 11:14:02 2024 +0100
@@ -211,7 +211,7 @@
     results: State.Results)     // finished results
 
   object State {
-    def inc_serial(serial: Long) = {
+    def inc_serial(serial: Long): Long = {
       require(serial < Long.MaxValue, "serial overflow")
       serial + 1
     }