ART: Move Signature to its own header

Reduce the dependencies on dex_file.h

Bug: 119869270
Test: mmma art
Change-Id: I1450fe2c3f4a7f5b535ed38cc19cb8a053228541
diff --git a/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index f3aefc2..2f37123 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -28,6 +28,7 @@
 #include "class_root.h"
 #include "common_runtime_test.h"
 #include "dex/dex_file_types.h"
+#include "dex/signature-inl.h"
 #include "dex/standard_dex_file.h"
 #include "entrypoints/entrypoint_utils-inl.h"
 #include "experimental_flags.h"