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)
215099e03f