ART: Wire up DexToDexCompiler without extern

The extern declaration actually had the wrong signature, a parameter
was missing, and we never noticed.

Remove the function field and extern. Add a header for the compiler.
Use the header file.

Change-Id: Ia4d4dd86211da6045709a45c7bf8430471d1b62b
diff --git a/compiler/jit/jit_compiler.cc b/compiler/jit/jit_compiler.cc
index c95bac2..4215f3c 100644
--- a/compiler/jit/jit_compiler.cc
+++ b/compiler/jit/jit_compiler.cc
@@ -153,7 +153,7 @@
   CompiledMethod* compiled_method = nullptr;
   {
     TimingLogger::ScopedTiming t2("Compiling", &logger);
-    compiled_method = compiler_driver_->CompileMethod(self, method);
+    compiled_method = compiler_driver_->CompileArtMethod(self, method);
   }
   {
     TimingLogger::ScopedTiming t2("TrimMaps", &logger);