Include configuration file for all headers

Include the configuration file for all headers that were not
included.
diff --git a/include/mbedtls/ecdh.h b/include/mbedtls/ecdh.h
index 99cfde0..d16bad2 100644
--- a/include/mbedtls/ecdh.h
+++ b/include/mbedtls/ecdh.h
@@ -33,6 +33,12 @@
 #ifndef MBEDTLS_ECDH_H
 #define MBEDTLS_ECDH_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include "ecp.h"
 
 #ifdef __cplusplus