stackleak: add declarations for global functions

With -Wmissing-prototypes enabled, the stackleak code produces a couple of
warnings that have no declarations because they are only called from assembler:

stackleak.c:127:25: error: no previous prototype for 'stackleak_erase' [-Werror=missing-prototypes]
stackleak.c:139:25: error: no previous prototype for 'stackleak_erase_on_task_stack' [-Werror=missing-prototypes]
stackleak.c:151:25: error: no previous prototype for 'stackleak_erase_off_task_stack' [-Werror=missing-prototypes]
stackleak.c:159:49: error: no previous prototype for 'stackleak_track_stack' [-Werror=missing-prototypes]

Add declarations to the stackleak header to shut up the warnings.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231108125843.3806765-7-arnd@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

include/linux/stackleak.h

@@ -14,6 +14,7 @@
 
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 #include <asm/stacktrace.h>
+#include <linux/linkage.h>
 
 /*
  * The lowest address on tsk's stack which we can plausibly erase.
@@ -76,6 +77,11 @@ static inline void stackleak_task_init(struct task_struct *t)
 # endif
 }
 
+asmlinkage void noinstr stackleak_erase(void);
+asmlinkage void noinstr stackleak_erase_on_task_stack(void);
+asmlinkage void noinstr stackleak_erase_off_task_stack(void);
+void __no_caller_saved_registers noinstr stackleak_track_stack(void);
+
 #else /* !CONFIG_GCC_PLUGIN_STACKLEAK */
 static inline void stackleak_task_init(struct task_struct *t) { }
 #endif