WMake: Avoid case collisions for install_prep

Extend ac77c23f9e (NMake,JOM,Borland: Avoid case collisions for
test_prep targets, 2026-10-06) to Watcom WMake, whose target names are
also case-insensitive.

Source/cmGlobalWatcomWMakeGenerator.h

@@ -54,6 +54,8 @@ public:
 
   bool IsGNUMakeJobServerAware() const override { return false; }
 
+  bool TargetNamesAreCaseInsensitive() const override { return true; }
+
   std::string GetShortBinaryOutputDir() const override;
 
 protected: