ART: Small iwyu

ArtField doesn't need dex_file_types. This in turn exposes a
missing include in modifiers.

Test: mmma art
Change-Id: I318e2919121f350a0447f6fc0814d81779fd9cc5
diff --git a/libdexfile/dex/modifiers.h b/libdexfile/dex/modifiers.h
index 0c79c96..32c06ce 100644
--- a/libdexfile/dex/modifiers.h
+++ b/libdexfile/dex/modifiers.h
@@ -17,6 +17,8 @@
 #ifndef ART_LIBDEXFILE_DEX_MODIFIERS_H_
 #define ART_LIBDEXFILE_DEX_MODIFIERS_H_
 
+#include <string>
+
 #include <stdint.h>
 
 namespace art {