firmware: turris-mox-rwtm: fix building without CONFIG_KEYS

"struct key" is defined conditionally, so the code referencing it
must be made conditional as well:

In file included from drivers/firmware/turris-mox-rwtm.c:29:
include/linux/turris-signing-key.h: In function 'turris_signing_key_get_dev':
include/linux/turris-signing-key.h:26:19: error: invalid use of undefined type 'const struct key'
   26 |         return key->payload.data[1];
      |                   ^~

Signed-off-by: Arnd Bergmann <[email protected]>

Author: arndb

include/linux/turris-signing-key.h

@@ -11,6 +11,7 @@
 
 struct device;
 
+#ifdef CONFIG_KEYS
 struct turris_signing_key_subtype {
 	u16 key_size;
 	u8 data_size;
@@ -29,5 +30,6 @@ static inline struct device *turris_signing_key_get_dev(const struct key *key)
 int
 devm_turris_signing_key_create(struct device *dev, const struct turris_signing_key_subtype *subtype,
 			       const char *desc);
+#endif
 
 #endif /* __TURRIS_SIGNING_KEY_H */