Create a header file for ECP internal functions

This header file will contain declarations of functions that are not
part of the public ABI/API, and must not be called from other modules,
but can be called from unit tests.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index 6a005d5..b07d6a2 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -77,6 +77,8 @@
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
 
+#include "ecp_invasive.h"
+
 #include <string.h>
 
 #if !defined(MBEDTLS_ECP_ALT)