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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/drivers/slirp_kern.c

···
 	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

···
 #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

···
 }
 
 void um_free_irq(int irq, void *dev_id);
+void free_irqs(void);
 #endif

arch/um/kernel/irq.c

···
 #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 -