90xbrlapi: Fix path to xbrlapi program
90xbrlapi: Fix path to xbrlapi program

since program_directory may be "${exec_prefix}/bin" and exec_prefix may be
${prefix}.

brltty/brltty,brltty/brltty,brltty/brltty,brltty/brltty,brltty/brltty,brltty/brltty