clarified signature: fewer warnings in IntelliJ IDEA;

--- a/src/Pure/Admin/component_csdp.scala	Wed Apr 09 22:23:59 2025 +0200
+++ b/src/Pure/Admin/component_csdp.scala	Wed Apr 09 22:29:11 2025 +0200
@@ -53,7 +53,7 @@
     target_dir: Path = Path.current,
     mingw: MinGW = MinGW.none
   ): Unit = {
-    mingw.check
+    mingw.check()
 
     Isabelle_System.with_tmp_dir("build") { tmp_dir =>
       /* component */

--- a/src/Pure/Admin/component_verit.scala	Wed Apr 09 22:23:59 2025 +0200
+++ b/src/Pure/Admin/component_verit.scala	Wed Apr 09 22:29:11 2025 +0200
@@ -19,7 +19,7 @@
     target_dir: Path = Path.current,
     mingw: MinGW = MinGW.none
   ): Unit = {
-    mingw.check
+    mingw.check()
 
     Isabelle_System.with_tmp_dir("build") { tmp_dir =>
       /* component */

--- a/src/Pure/System/mingw.scala	Wed Apr 09 22:23:59 2025 +0200
+++ b/src/Pure/System/mingw.scala	Wed Apr 09 22:29:11 2025 +0200
@@ -51,7 +51,7 @@
     else if (root.isEmpty) error("Windows platform requires msys/mingw root specification")
     else root.get
 
-  def check: Unit = {
+  def check(): Unit = {
     if (Platform.is_windows) {
       get_root
       try { require(Isabelle_System.bash(bash_script("uname -s")).check.out.startsWith("MSYS")) }