um: fix 3 instances of -Wmissing-prototypes

Fixes the following build errors observed from W=1 builds:
  arch/um/drivers/xterm_kern.c:35:5: warning: no previous prototype for
  function 'xterm_fd' [-Wmissing-prototypes]
  35 | int xterm_fd(int socket, int *pid_out)
     |     ^
  arch/um/drivers/xterm_kern.c:35:1: note: declare 'static' if the
  function is not intended to be used outside of this translation unit
  35 | int xterm_fd(int socket, int *pid_out)
     | ^
     | static
  arch/um/drivers/chan_kern.c:183:6: warning: no previous prototype for
  function 'free_irqs' [-Wmissing-prototypes]
  183 | void free_irqs(void)
      |      ^
  arch/um/drivers/chan_kern.c:183:1: note: declare 'static' if the
  function is not intended to be used outside of this translation unit
  183 | void free_irqs(void)
      | ^
      | static
  arch/um/drivers/slirp_kern.c:18:6: warning: no previous prototype for
  function 'slirp_init' [-Wmissing-prototypes]
  18 | void slirp_init(struct net_device *dev, void *data)
     |      ^
  arch/um/drivers/slirp_kern.c:18:1: note: declare 'static' if the
  function is not intended to be used outside of this translation unit
  18 | void slirp_init(struct net_device *dev, void *data)
     | ^
     | static

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308081050.sZEw4cQ5-lkp@intel.com/


Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>