Include configuration file for all headers

Include the configuration file for all headers that were not
included.
diff --git a/include/mbedtls/ssl_cache.h b/include/mbedtls/ssl_cache.h
index ec081e6..52ba094 100644
--- a/include/mbedtls/ssl_cache.h
+++ b/include/mbedtls/ssl_cache.h
@@ -24,6 +24,12 @@
 #ifndef MBEDTLS_SSL_CACHE_H
 #define MBEDTLS_SSL_CACHE_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include "ssl.h"
 
 #if defined(MBEDTLS_THREADING_C)