openrisc: Add prototype for show_registers to processor.h

When compiling with W=1 enabling -Wmissing-prototypes the compiler
warns:

  arch/openrisc/kernel/traps.c:67:6: error: no previous prototype for 'show_registers' [-Werror=missing-prototypes]

Fix by adding the prototype to the appropriate header file and including
the header file in the appropriate C files.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/linux-kernel/20230810141947.1236730-17-arnd@kernel.org/Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/include/asm/processor.h

@@ -73,6 +73,7 @@ struct thread_struct {
 
 void start_thread(struct pt_regs *regs, unsigned long nip, unsigned long sp);
 unsigned long __get_wchan(struct task_struct *p);
+void show_registers(struct pt_regs *regs);
 
 #define cpu_relax()     barrier()
 

arch/openrisc/kernel/process.c

@@ -119,8 +119,6 @@ void flush_thread(void)
 
 void show_regs(struct pt_regs *regs)
 {
-	extern void show_registers(struct pt_regs *regs);
-
 	show_regs_print_info(KERN_DEFAULT);
 	/* __PHX__ cleanup this mess */
 	show_registers(regs);

arch/openrisc/kernel/traps.c

@@ -31,6 +31,7 @@
 #include <linux/uaccess.h>
 
 #include <asm/io.h>
+#include <asm/processor.h>
 #include <asm/unwinder.h>
 #include <asm/sections.h>