cmGeneratorTarget: fix misplaced header

In commit b36d1bdd9d (Single location for cmProp typedef, 2020-05-30)
we accidentally left inclusion of `cmProperty.h` out of the main
block of includes.

Source/cmGeneratorTarget.h

@@ -18,6 +18,7 @@
 #include "cmLinkItem.h"
 #include "cmListFileCache.h"
 #include "cmPolicies.h"
+#include "cmProperty.h"
 #include "cmStateTypes.h"
 
 class cmComputeLinkInformation;
@@ -31,8 +32,6 @@ class cmTarget;
 struct cmGeneratorExpressionContext;
 struct cmGeneratorExpressionDAGChecker;
 
-#include "cmProperty.h"
-
 class cmGeneratorTarget
 {
 public: