module: Only declare set_module_sig_enforced when CONFIG_MODULE_SIG=y

Currently if set_module_sig_enforced is called with CONFIG_MODULE_SIG=n
e.g. [1], it can lead to a linking error,

    ld: security/integrity/ima/ima_appraise.o: in function `ima_appraise_measurement':
    security/integrity/ima/ima_appraise.c:587:(.text+0xbbb): undefined reference to `set_module_sig_enforced'

This happens because the actual implementation of
set_module_sig_enforced comes from CONFIG_MODULE_SIG but both the
function declaration and the empty stub definition are tied to
CONFIG_MODULES.

So bind set_module_sig_enforced to CONFIG_MODULE_SIG instead. This
allows (future) users to call set_module_sig_enforced directly without
the "if IS_ENABLED(CONFIG_MODULE_SIG)" safeguard.

Note this issue hasn't caused a real problem because all current callers
of set_module_sig_enforced e.g. security/integrity/ima/ima_efi.c
use "if IS_ENABLED(CONFIG_MODULE_SIG)" safeguard.

[1] https://lore.kernel.org/lkml/20250928030358.3873311-1-coxu@redhat.com/

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510030029.VRKgik99-lkp@intel.com/
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Author: coiby

include/linux/module.h

@@ -764,8 +764,6 @@ static inline bool is_livepatch_module(struct module *mod)
 #endif
 }
 
-void set_module_sig_enforced(void);
-
 void module_for_each_mod(int(*func)(struct module *mod, void *data), void *data);
 
 #else /* !CONFIG_MODULES... */
@@ -860,10 +858,6 @@ static inline bool module_requested_async_probing(struct module *module)
 }
 
 
-static inline void set_module_sig_enforced(void)
-{
-}
-
 /* Dereference module function descriptor */
 static inline
 void *dereference_module_function_descriptor(struct module *mod, void *ptr)
@@ -919,6 +913,8 @@ static inline bool retpoline_module_ok(bool has_retpoline)
 #ifdef CONFIG_MODULE_SIG
 bool is_module_sig_enforced(void);
 
+void set_module_sig_enforced(void);
+
 static inline bool module_sig_ok(struct module *module)
 {
 	return module->sig_ok;
@@ -929,6 +925,10 @@ static inline bool is_module_sig_enforced(void)
 	return false;
 }
 
+static inline void set_module_sig_enforced(void)
+{
+}
+
 static inline bool module_sig_ok(struct module *module)
 {
 	return true;