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/include/mbedtls/config.h b/include/mbedtls/config.h
index 4aec9ac..b80667a 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1129,6 +1129,17 @@
 #define MBEDTLS_SSL_ALL_ALERT_MESSAGES
 
 /**
+ * \def MBEDTLS_SSL_ASYNC_PRIVATE
+ *
+ * Enable asynchronous external private key operations in SSL. This allows
+ * you to configure an SSL connection to call an external cryptographic
+ * module to perform private key operations instead of performing the
+ * operation inside the library.
+ *
+ */
+//#define MBEDTLS_SSL_ASYNC_PRIVATE
+
+/**
  * \def MBEDTLS_SSL_DEBUG_ALL
  *
  * Enable the debug messages in SSL module for all issues.
@@ -2473,17 +2484,6 @@
 #define MBEDTLS_SHA512_C
 
 /**
- * \def MBEDTLS_SSL_ASYNC_PRIVATE_C
- *
- * Enable asynchronous external private key operations in SSL. This allows
- * you to configure an SSL connection to call an external cryptographic
- * module to perform private key operations instead of performing the
- * operation inside the library.
- *
- */
-//#define MBEDTLS_SSL_ASYNC_PRIVATE_C
-
-/**
  * \def MBEDTLS_SSL_CACHE_C
  *
  * Enable simple SSL cache implementation.