lzx-decompress.c: Fix alignment bug

lzx-decompress.c:  Fix alignment bug

The pretree decode table was not declared to be 16 byte aligned, as
expected by make_huffman_decode_table().

This bug had no effect if the compiler aligned this table on a 16 byte
boundary anyway, but if not it caused a segmentation fault on x86 platforms
where the SSE2 instructions were available, since in that case stores
requiring 16 byte alignment are used to fill in table entries.