Fix compilation with ENABLE_BACKTRACE=OFF

  src/lua/init.c: In function ‘tarantool_panic_handler’:
  src/lua/init.c:321:2: error: implicit declaration of function ‘print_backtrace’ [-Werror=implicit-function-declaration]
    print_backtrace();
    ^~~~~~~~~~~~~~~

  src/lua/fiber.c:244:1: error: ‘lbox_fiber_statof_bt’ defined but not used [-Werror=unused-function]
   lbox_fiber_statof_bt(struct fiber *f, void *cb_ctx)
   ^~~~~~~~~~~~~~~~~~~~

src/lua/fiber.c

@@ -240,11 +240,13 @@ lbox_fiber_statof(struct fiber *f, void *cb_ctx, bool backtrace)
 	return 0;
 }
 
+#ifdef ENABLE_BACKTRACE
 static int
 lbox_fiber_statof_bt(struct fiber *f, void *cb_ctx)
 {
 	return lbox_fiber_statof(f, cb_ctx, true);
 }
+#endif
 
 static int
 lbox_fiber_statof_nobt(struct fiber *f, void *cb_ctx)

src/lua/init.c

@@ -318,7 +318,9 @@ tarantool_lua_setpaths(struct lua_State *L)
 static int
 tarantool_panic_handler(lua_State *L) {
 	const char *problem = lua_tostring(L, -1);
+#ifdef ENABLE_BACKTRACE
 	print_backtrace();
+#endif
 	say_crit("%s", problem);
 	int level = 1;
 	lua_Debug ar;