Include configuration file to headers

Add inclusion to configration file in header files,
instead of relying on other header files to include
the configuration file. This issue resolves #1371
diff --git a/include/mbedtls/ecp_internal.h b/include/mbedtls/ecp_internal.h
index 1804069..7625ed4 100644
--- a/include/mbedtls/ecp_internal.h
+++ b/include/mbedtls/ecp_internal.h
@@ -61,6 +61,12 @@
 #ifndef MBEDTLS_ECP_INTERNAL_H
 #define MBEDTLS_ECP_INTERNAL_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #if defined(MBEDTLS_ECP_INTERNAL_ALT)
 
 /**