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.cc b/runtime/class_linker.cc
index c22a5cb..5d1f20c 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -61,6 +61,7 @@
 #include "dex/dex_file-inl.h"
 #include "dex/dex_file_exception_helpers.h"
 #include "dex/dex_file_loader.h"
+#include "dex/signature-inl.h"
 #include "dex/utf.h"
 #include "entrypoints/entrypoint_utils.h"
 #include "entrypoints/runtime_asm_entrypoints.h"