ART: Forward-declare ClassIterator and Signature in dex_file.h (iwyu)

Test: mmma art
Change-Id: I9385dd02db52ef10bf748cde118c1573733faffb
diff --git a/libdexfile/dex/dex_file-inl.h b/libdexfile/dex/dex_file-inl.h
index 15ba9cc..b777ef7 100644
--- a/libdexfile/dex/dex_file-inl.h
+++ b/libdexfile/dex/dex_file-inl.h
@@ -27,6 +27,7 @@
 #include "compact_dex_file.h"
 #include "dex_instruction_iterator.h"
 #include "invoke_type.h"
+#include "signature.h"
 #include "standard_dex_file.h"
 
 namespace art {
diff --git a/libdexfile/dex/dex_file.h b/libdexfile/dex/dex_file.h
index 0f1d8b3..5cfbc1f 100644
--- a/libdexfile/dex/dex_file.h
+++ b/libdexfile/dex/dex_file.h
@@ -27,22 +27,22 @@
 #include "base/globals.h"
 #include "base/macros.h"
 #include "base/value_object.h"
-#include "class_iterator.h"
 #include "dex_file_structs.h"
 #include "dex_file_types.h"
 #include "jni.h"
 #include "modifiers.h"
-#include "signature.h"
 
 namespace art {
 
 class ClassDataItemIterator;
+class ClassIterator;
 class CompactDexFile;
 class DexInstructionIterator;
 enum InvokeType : uint32_t;
 template <typename Iter> class IterationRange;
 class MemMap;
 class OatDexFile;
+class Signature;
 class StandardDexFile;
 class ZipArchive;