env_callback.h: spl: mark callback as maybe_unused

When static inline is used in a header file the function
should preferably be inlined and if not possible made a
static function. When declared inside a c file there is a
static function, which might be inlined. Since SPL uses a
define to declare the static inline it becomes part of the
c file although it is declared in a header and clang will
warn that you have introduced unused static functions. Add
maybe_unused to prevent such warnings.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>

include/env_callback.h

@@ -60,7 +60,7 @@ void env_callback_init(ENTRY *var_entry);
  */
 #ifdef CONFIG_SPL_BUILD
 #define U_BOOT_ENV_CALLBACK(name, callback) \
-	static inline void _u_boot_env_noop_##name(void) \
+	static inline __maybe_unused void _u_boot_env_noop_##name(void) \
 	{ \
 		(void)callback; \
 	}