add forward declaration in rust_values.h

Target class is referenced, but header is not included.
This is to fix
http://ci.chromium.org/b/8758787771688889681
for
https://gn-review.git.corp.google.com/c/gn/+/16440/2

Change-Id: I0b516a4c8d8cb845951acc77b03664f11f91a8d1
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16460
Reviewed-by: David Turner <digit@google.com>
Commit-Queue: David Turner <digit@google.com>

src/gn/rust_values.h

diff --git a/src/gn/rust_values.h b/src/gn/rust_values.h
index 52a7623..e958295 100644
--- a/src/gn/rust_values.h
+++ b/src/gn/rust_values.h
@@ -11,6 +11,8 @@
 #include "gn/label.h"
 #include "gn/source_file.h"
 
+class Target;
+
 // Holds the values (outputs, args, script name, etc.) for either an action or
 // an action_foreach target.
 class RustValues {