cmMakefile: Add missing initializer for recently added member

In commit 2f1ffa003c (find_package: Add support for default GLOBAL
imported targets, 2022-03-10) we added a field without an initializer.
This was exposed by a few failures in nightly testing.  Previously it
worked only by chance that the member's memory had suitable values.

Source/cmMakefile.h

@@ -1162,5 +1162,5 @@ private:
   std::set<std::string> WarnedCMP0074;
   bool IsSourceFileTryCompile;
   mutable bool SuppressSideEffects;
-  ImportedTargetScope CurrentImportedTargetScope;
+  ImportedTargetScope CurrentImportedTargetScope = ImportedTargetScope::Local;
 };