[basic.lookup.argdep]/5 add export to apply()

fix #2968

Author: onihusube

source/basic.tex

@@ -1914,7 +1914,7 @@
 import M;
 export R::X make();
 namespace R { static int g(X); }
-template<typename T, typename U> void apply(T t, U u) {
+export template<typename T, typename U> void apply(T t, U u) {
   f(t, u);
   g(t);
 }