wasm: Define jit_exec only if any JIT is enabled

```
../src/vm.c:566:1: warning: unused function 'jit_exec' [-Wunused-function]
  566 | jit_exec(rb_execution_context_t *ec)
      | ^~~~~~~~
```

Author: nobu

vm.c

@@ -561,6 +561,7 @@ zjit_compile(rb_execution_context_t *ec)
 
 static inline void zjit_materialize_frames(rb_control_frame_t *cfp);
 
+#if USE_YJIT || USE_ZJIT
 // Execute JIT code compiled by yjit_compile() or zjit_compile()
 static inline VALUE
 jit_exec(rb_execution_context_t *ec)
@@ -595,7 +596,6 @@ jit_exec(rb_execution_context_t *ec)
     return Qundef;
 }
 
-#if USE_YJIT || USE_ZJIT
 // Generate JIT code that supports the following kind of ISEQ entry:
 //   * The first ISEQ pushed by vm_exec_handle_exception. The frame would
 //     point to a location specified by a catch table, and it doesn't have