FileUtil: Don't try to retrieve size for FIFO pipe

[itsmattkc]

This is an extremely small patch that fixes an exception thrown if you try to use POSIX pipes as control inputs (as detailed in this wiki article).

On the current master, if you have a POSIX pipe in Dolphin's "Pipes" folder, it will crash on startup with the following message:

terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'
  what():  filesystem error: cannot get file size: Operation not supported [<filename>]

This happens because ScanDirectoryTree attempts to get the size of all non-directories, which as the exception explains is not valid for FIFO pipes.

This patch adds an additional check for is_fifo() in addition to the existing check for is_directory(), returning 0 for the size in both cases.