ART: Forward-declare Signature in art_method.h (iwyu)

Test: mmma art
Change-Id: I16e2083db5b52c9373350a1072550d1e83355ab5
diff --git a/runtime/art_method.h b/runtime/art_method.h
index e4c21f0..23c1fe9 100644
--- a/runtime/art_method.h
+++ b/runtime/art_method.h
@@ -31,7 +31,6 @@
 #include "dex/dex_file_structs.h"
 #include "dex/modifiers.h"
 #include "dex/primitive.h"
-#include "dex/signature.h"
 #include "gc_root.h"
 #include "obj_ptr.h"
 #include "offsets.h"
@@ -51,6 +50,7 @@
 class ProfilingInfo;
 class ScopedObjectAccessAlreadyRunnable;
 class ShadowFrame;
+class Signature;
 
 namespace mirror {
 class Array;