include log1p_exp in prim lub_constrain

Author: spinkney

stan/math/prim/constraint/lub_constrain.hpp

@@ -13,6 +13,7 @@
 #include <stan/math/prim/fun/inv_logit.hpp>
 #include <stan/math/prim/fun/log.hpp>
 #include <stan/math/prim/fun/log1p.hpp>
+#include <stan/math/prim/fun/log1p_exp.hpp>
 #include <stan/math/prim/fun/multiply.hpp>
 #include <stan/math/prim/fun/subtract.hpp>
 #include <stan/math/prim/fun/sum.hpp>