Move declarations of testing-only base64 functions to their own header

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/base64.c b/library/base64.c
index 96c94d1..085c71f 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -22,6 +22,7 @@
 #if defined(MBEDTLS_BASE64_C)
 
 #include "mbedtls/base64.h"
+#include "base64_invasive.h"
 
 #include <stdint.h>