Fix a copy-paste error in define comment

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/tests/src/external_timing/timing_alt.h b/tests/src/external_timing/timing_alt.h
index 204dbe8..82e8c8b 100644
--- a/tests/src/external_timing/timing_alt.h
+++ b/tests/src/external_timing/timing_alt.h
@@ -127,4 +127,4 @@
 #ifdef __cplusplus
 }
 
-#endif /* FAKE_EXTERNAL_RNG_FOR_TEST_H */
+#endif /* EXTERNAL_TIMING_FOR_TEST_H */