pttd: fix memory leak in yasm_alloc()

In yasm_alloc(), we allocate y->fileroot, yet we forget to free it again in
yasm_free().  Fix it.

Change-Id: Ibd68836602d7bfccbe2c15ef0cfe33b807827cf8
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>