U.S. investors spooked by wild swings in the foreign exchange market are piling into exchange-traded funds that strip out the local currency on their international equity portfolios, making them one of the most sought-after financial products in 2015. With the dollar having rallied more than 19 percent since the beginning of 2014, investors are seeing gains in overseas stock markets eaten up by losses against the greenback. “People are voting with their feet,” said Luciano Siracusano, chief investment strategist for WisdomTree Investments in New York.