arm64: hide unused is_valid_bugaddr()

When generic BUG() support is disabled, this function has no declaration
and no callers but causes a W=1 warning:

arch/arm64/kernel/traps.c:950:5: error: no previous prototype for 'is_valid_bugaddr' [-Werror=missing-prototypes]

Add an #ifdef that matches the one around the declaration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230516160642.523862-10-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: arndb

arch/arm64/kernel/traps.c

@@ -947,7 +947,7 @@ void do_serror(struct pt_regs *regs, unsigned long esr)
 }
 
 /* GENERIC_BUG traps */
-
+#ifdef CONFIG_GENERIC_BUG
 int is_valid_bugaddr(unsigned long addr)
 {
 	/*
@@ -959,6 +959,7 @@ int is_valid_bugaddr(unsigned long addr)
 	 */
 	return 1;
 }
+#endif
 
 static int bug_handler(struct pt_regs *regs, unsigned long esr)
 {