schema CHANGE move lysc_ext_substmt() into general public API

The function seems useless in (public) extensions plugins API, the
plugins themselves should know best the content of the substatements
information and be able to access them. The helper function is useful
for libyang internal code and libyang users.
diff --git a/src/schema_compile.c b/src/schema_compile.c
index 3588a7d..d725efb 100644
--- a/src/schema_compile.c
+++ b/src/schema_compile.c
@@ -143,7 +143,7 @@
     return orig;
 }
 
-LY_ERR
+API LY_ERR
 lysc_ext_substmt(const struct lysc_ext_instance *ext, enum ly_stmt substmt, void **instance_p, enum ly_stmt_cardinality *cardinality_p)
 {
     LY_ARRAY_COUNT_TYPE u;