LoongArch: ftrace: Fix build error if DYNAMIC_FTRACE_WITH_REGS is not set

We can see the following build error if CONFIG_DYNAMIC_FTRACE_WITH_REGS
is not set on LoongArch:

arch/loongarch/kernel/ftrace_dyn.c: In function ‘ftrace_make_call’:
arch/loongarch/kernel/ftrace_dyn.c:167:23: error: implicit declaration of function ‘__get_mod’
  167 |                 ret = __get_mod(&mod, pc);
      |                       ^~~~~~~~~
arch/loongarch/kernel/ftrace_dyn.c:171:24: error: implicit declaration of function ‘get_plt_addr’
  171 |                 addr = get_plt_addr(mod, addr);
      |                        ^~~~~~~~~~~~

The reason is that the __get_mod() and get_plt_addr() may be called in
ftrace_make_{call,nop}.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>