platform: expose nm_link_type_to_string() function

Given the name nm_link_type_to_string(), we would not expect
to find it in nm-linux-platform.c. It either should be named
nm_platform_link_type_to_string() and be put in a new
nm-platform-utils.c file, or it should be named
nm_utils_link_type_to_string() and be put in NetworkManagerUtils.h.

For now, just leave it here.

src/platform/nm-linux-platform.c

@@ -913,7 +913,7 @@ nm_link_type_to_rtnl_type_string (NMLinkType type)
 	g_return_val_if_reached (NULL);
 }
 
-static const char *
+const char *
 nm_link_type_to_string (NMLinkType type)
 {
 	int i;