ART: Forward-declare Context

Move deletion to thread.cc, which allows to forward-declare
Context.

Bug: 119869270
Test: mmma art
Change-Id: Ifd9006cd3457d4ab708a289a7f6e1cdf2591c596
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index f30ba0c..b925b96 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -42,6 +42,7 @@
 #include "aot_class_linker.h"
 #include "arch/arm/registers_arm.h"
 #include "arch/arm64/registers_arm64.h"
+#include "arch/context.h"
 #include "arch/instruction_set_features.h"
 #include "arch/mips/registers_mips.h"
 #include "arch/mips64/registers_mips64.h"