Remove incorrect attributes from posix_memalign(3) declaration.

Both alloc_align and alloc_size can't be used when the function
returns a pointer to memory. This fixes breakage when building with
clang 3.4:

In file included from /usr/src/svn/usr.sbin/bhyve/atkbdc.c:40:
/usr/include/stdlib.h:176:6: error: 'alloc_size' attribute only
applies to functions that return a pointer [-Werror,-Wignored-attributes]

Pointed out by: ngie, cem
Approved by: re (gjb)