kasan: add kasan_tag_mismatch prototype

The kasan sw-tags implementation contains one function that is only called
from assembler and has no prototype in a header.  This causes a W=1
warning:

mm/kasan/sw_tags.c:171:6: warning: no previous prototype for 'kasan_tag_mismatch' [-Wmissing-prototypes]
  171 | void kasan_tag_mismatch(unsigned long addr, unsigned long access_info,

Add a prototype in the local header to get a clean build.

Link: https://lkml.kernel.org/r/20230509145735.9263-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kasan/kasan.h

@@ -646,4 +646,7 @@ void *__hwasan_memset(void *addr, int c, size_t len);
 void *__hwasan_memmove(void *dest, const void *src, size_t len);
 void *__hwasan_memcpy(void *dest, const void *src, size_t len);
 
+void kasan_tag_mismatch(unsigned long addr, unsigned long access_info,
+			unsigned long ret_ip);
+
 #endif /* __MM_KASAN_KASAN_H */