apply hidden visibility to tlsdesc accessor functions

these functions are never called directly; only their addresses are
used, so PLT indirections should never happen unless a broken
application tries to redefine them, but it's still best to make them
hidden.
diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index 9f91a73..ccd526f 100644
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -250,6 +250,7 @@
 	return def;
 }
 
+__attribute__((__visibility__("hidden")))
 ptrdiff_t __tlsdesc_static(), __tlsdesc_dynamic();
 
 static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stride)