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/dex2oat/linker/image_test.h b/dex2oat/linker/image_test.h
index 70a22b5..8c9dfb8 100644
--- a/dex2oat/linker/image_test.h
+++ b/dex2oat/linker/image_test.h
@@ -36,6 +36,7 @@
 #include "compiler_callbacks.h"
 #include "debug/method_debug_info.h"
 #include "dex/quick_compiler_callbacks.h"
+#include "dex/signature-inl.h"
 #include "driver/compiler_driver.h"
 #include "driver/compiler_options.h"
 #include "gc/space/image_space.h"