Add blueprint:mutated to sysprop properties

Because this flag is used internally by sysprop_library.

Bug: N/A
Test: m -j
Test: build cc_library with setting sysprop property and fail
Change-Id: I478889b8923f99407a067e5b9efee9322c3e5ae8
diff --git a/cc/library.go b/cc/library.go
index c23f26b..cf18ebf 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -70,7 +70,7 @@
 	Sysprop struct {
 		// Whether platform owns this sysprop library.
 		Platform *bool
-	}
+	} `blueprint:"mutated"`
 
 	Static_ndk_lib *bool