lib/defines.h: Don't wrap #undef in #ifdef

lib/defines.h: Don't wrap #undef in #ifdef

ISO C guarantees that #undef is a no-op if there is no such macro.

C11::6.10.3.5p2:
> A preprocessing directive of the form
>
>       # undef identifier new-line
>
> causes the specified identifier no longer to be defined as a macro
> name.  It is ignored if the specified identifier is not currently
> defined as a macro name.

Link: <http://port70.net/~nsz/c/c11/n1570.html#6.10.3.5p2>
Signed-off-by: Alejandro Colomar <alx@kernel.org>