pdf: fixes compiler error under Windows.
pdf: fixes compiler error under Windows.

fpgOpenURL() will work in this case for both Linux and Windows, but it is
not the ideal solution. As the TODO entry says, we ultimately need to
implement a fpgViewFile() function.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI