Use -Wall compiler warning level

Sets the C and C++ compiler warning level to -Wall. Fixes for
warnings judged to be non-functional are included in this commit.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I783ff274b7fbd802bc25074953e2c2f287114605
diff --git a/components/service/smm_variable/backend/variable_checker.c b/components/service/smm_variable/backend/variable_checker.c
index 81a41d0..81a6cc6 100644
--- a/components/service/smm_variable/backend/variable_checker.c
+++ b/components/service/smm_variable/backend/variable_checker.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Arm Limited. All rights reserved.
+ * Copyright (c) 2021-2022, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -53,6 +53,8 @@
 	uint32_t attributes,
 	size_t data_size)
 {
+	(void)attributes;
+
 	if (constraints->property & VAR_CHECK_VARIABLE_PROPERTY_READ_ONLY)
 		return EFI_WRITE_PROTECTED;