This avoids uncaught exceptions when the 'strip' phase would call these
procedures on symlinks to directories, such as 'lib/terminfo' in
ncurses (see <http://hydra.gnu.org/build/167310/nixlog/1/tail-reload>.)
* guix/build/utils.scm (file-header-match): Catch 'system-error', and
return #f upon EISDIR.