Rename plain c disable option

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aesni.h b/library/aesni.h
index fa1f369..341350a 100644
--- a/library/aesni.h
+++ b/library/aesni.h
@@ -88,7 +88,7 @@
  *
  * \return         1 if CPU has support for the feature, 0 otherwise
  */
-#if !defined(MBEDTLS_AES_HAS_NO_BUILTIN)
+#if !defined(MBEDTLS_AES_HAS_NO_PLAIN_C)
 int mbedtls_aesni_has_support(unsigned int what);
 #else
 #define /* no-check-names */ mbedtls_aesni_has_support(what) 1