xtensa: processor.h: add init_arch() prototype

Add the prototype for init_arch() to asm/processor.h to prevent a
build warning:

arch/xtensa/kernel/setup.c:244:13: warning: no previous prototype for 'init_arch' [-Wmissing-prototypes]
  244 | void __init init_arch(bp_tag_t *bp_start)
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-6-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/processor.h

@@ -14,6 +14,8 @@
 
 #include <linux/compiler.h>
 #include <linux/stringify.h>
+
+#include <asm/bootparam.h>
 #include <asm/ptrace.h>
 #include <asm/types.h>
 #include <asm/regs.h>
@@ -217,6 +219,8 @@ struct mm_struct;
 
 extern unsigned long __get_wchan(struct task_struct *p);
 
+void init_arch(bp_tag_t *bp_start);
+
 #define KSTK_EIP(tsk)		(task_pt_regs(tsk)->pc)
 #define KSTK_ESP(tsk)		(task_pt_regs(tsk)->areg[1])