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>

Author: nickdesaulniers

arch/um/drivers/slirp_kern.c

@@ -15,7 +15,7 @@ struct slirp_init {
 	struct arg_list_dummy_wrapper argw;  /* XXX should be simpler... */
 };
 
-void slirp_init(struct net_device *dev, void *data)
+static void slirp_init(struct net_device *dev, void *data)
 {
 	struct uml_net_private *private;
 	struct slirp_data *spri;

arch/um/drivers/xterm_kern.c

@@ -9,6 +9,7 @@
 #include <asm/irq.h>
 #include <irq_kern.h>
 #include <os.h>
+#include "xterm.h"
 
 struct xterm_wait {
 	struct completion ready;

arch/um/include/shared/irq_kern.h

@@ -76,4 +76,5 @@ static inline bool um_irq_timetravel_handler_used(void)
 }
 
 void um_free_irq(int irq, void *dev_id);
+void free_irqs(void);
 #endif

arch/um/kernel/irq.c

@@ -23,8 +23,6 @@
 #include <linux/time-internal.h>
 
 
-extern void free_irqs(void);
-
 /* When epoll triggers we do not know why it did so
  * we can also have different IRQs for read and write.
  * This is why we keep a small irq_reg array for each fd -