fix(fconf): make struct fconf_populator static

In FCONF_REGISTER_POPULATOR macro, add static for the fconf_populator
struct. This avoids this kind of sparse warning:
plat/st/common/stm32mp_fconf_io.c:181:1: warning:
 symbol 'stm32mp_io__populator' was not declared. Should it be static?

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Icaa7da3079e82497e112585150f6348ef2ebf5e6
diff --git a/include/lib/fconf/fconf.h b/include/lib/fconf/fconf.h
index 131c542..3762021 100644
--- a/include/lib/fconf/fconf.h
+++ b/include/lib/fconf/fconf.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2020, ARM Limited. All rights reserved.
+ * Copyright (c) 2019-2022, ARM Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -21,7 +21,7 @@
  */
 #define FCONF_REGISTER_POPULATOR(config, name, callback)			\
 	__attribute__((used, section(".fconf_populator")))			\
-	const struct fconf_populator (name##__populator) = {			\
+	static const struct fconf_populator (name##__populator) = {		\
 		.config_type = (#config),					\
 		.info = (#name),						\
 		.populate = (callback)						\