ARM: dyndbg: allow including dyndbg.h in decompressor

After a change to linux/module.h, dyndbg.h is now included
indirectly from the decompressor for lz4 support, which in turn
causes a build failure on 32-bit Arm:

In file included from include/linux/module.h:30,
                 from arch/arm/boot/compressed/../../../../lib/lz4/lz4_decompress.c:39,
                 from arch/arm/boot/compressed/../../../../lib/decompress_unlz4.c:10,
                 from arch/arm/boot/compressed/decompress.c:59:
include/linux/dynamic_debug.h: In function 'ddebug_dyndbg_module_param_cb':
include/linux/dynamic_debug.h:307:14: error: implicit declaration of function 'strcmp' [-Werror=implicit-function-declaration]
  307 |         if (!strcmp(param, "dyndbg")) {
      |              ^~~~~~
include/linux/dynamic_debug.h:1:1: note: 'strcmp' is defined in header '<string.h>'; did you forget to '#include <string.h>'?
  +++ |+#include <string.h>

The decompressor has its own replacement for the linux/string.h contents,
so the normal declaration is not visible here. Since the function is
not actually called, it is sufficient to add a declaration, and this
is in fact the correct one as it matches the definition in
arch/arm/boot/compressed/string.c.

Fixes: 7deabd67 ("dyndbg: use the module notifier callbacks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jason Baron <jbaron@akamai.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

arch/arm/boot/compressed/decompress.c

@@ -31,6 +31,7 @@
 /* Not needed, but used in some headers pulled in by decompressors */
 extern char * strstr(const char * s1, const char *s2);
 extern size_t strlen(const char *s);
+extern int strcmp(const char *cs, const char *ct);
 extern int memcmp(const void *cs, const void *ct, size_t count);
 extern char * strchrnul(const char *, int);