Rename plain c disable option

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aesce.c b/library/aesce.c
index baa01db..982cad6 100644
--- a/library/aesce.c
+++ b/library/aesce.c
@@ -99,7 +99,7 @@
 #include <sys/auxv.h>
 #endif
 
-#if !defined(MBEDTLS_AES_HAS_NO_BUILTIN)
+#if !defined(MBEDTLS_AES_HAS_NO_PLAIN_C)
 /*
  * AES instruction support detection routine
  */