Suppress unused-function warning when OPT_THREADED_CODE != 1

vm_exec.c

@@ -15,6 +15,7 @@
 static void vm_analysis_insn(int insn);
 #endif
 
+MAYBE_UNUSED(static void vm_insns_counter_count_insn(int insn));
 #if USE_INSNS_COUNTER
 static size_t rb_insns_counter[VM_INSTRUCTION_SIZE];