Rename MBEDTLS_SSL_ASYNC_PRIVATE_C to MBEDTLS_SSL_ASYNC_PRIVATE

This is an optional feature, not a module of its own, so don't call it
MBEDTLS_xxx_C and put it in the appropriate section of config.h.
diff --git a/library/version_features.c b/library/version_features.c
index c017c41..659b96d 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -402,6 +402,9 @@
 #if defined(MBEDTLS_SSL_ALL_ALERT_MESSAGES)
     "MBEDTLS_SSL_ALL_ALERT_MESSAGES",
 #endif /* MBEDTLS_SSL_ALL_ALERT_MESSAGES */
+#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
+    "MBEDTLS_SSL_ASYNC_PRIVATE",
+#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
     "MBEDTLS_SSL_DEBUG_ALL",
 #endif /* MBEDTLS_SSL_DEBUG_ALL */
@@ -648,9 +651,6 @@
 #if defined(MBEDTLS_SHA512_C)
     "MBEDTLS_SHA512_C",
 #endif /* MBEDTLS_SHA512_C */
-#if defined(MBEDTLS_SSL_ASYNC_PRIVATE_C)
-    "MBEDTLS_SSL_ASYNC_PRIVATE_C",
-#endif /* MBEDTLS_SSL_ASYNC_PRIVATE_C */
 #if defined(MBEDTLS_SSL_CACHE_C)
     "MBEDTLS_SSL_CACHE_C",
 #endif /* MBEDTLS_SSL_CACHE_C */