Add __maybe_unused to variable used only in assert()

Fixes a compilation warning/error when tests are build with NDEBUG
defined (make CPPFLAGS=-DNDEBUG) if OP-TEE pull request #910 [1] is
merged.

[1] https://github.com/OP-TEE/optee_os/pull/910

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: David Brown <david.brown@linaro.org>
diff --git a/ta/os_test/tb_gcd.c b/ta/os_test/tb_gcd.c
index 46392ff..bc6ac2d 100644
--- a/ta/os_test/tb_gcd.c
+++ b/ta/os_test/tb_gcd.c
@@ -26,6 +26,7 @@
  */
 #include "testframework.h"
 #include <assert.h>
+#include <compiler.h>
 
 #define TEST_GCD(stra, strb)                  \
 do {                                          \
@@ -119,7 +120,7 @@
 
 static void test_relative_prime(void)
 {
-	bool res;
+	bool res __maybe_unused;
 
 	DEF_BIGINT(x, 2048);
 	DEF_BIGINT(y, 2048);