package/elfutils: fix dependency on C library

As Thomas puts it:

    The comment can only be visible when a toolchain that is *not*
    uclibc and *not* glibc is used. I.e, the comment is now only visible
    when musl is used. Which is not what we want.

Indeed, I completely borked the conditions. When a glibc or uClibc
toolchain is selected, the comment is entirely hidden, and we don;t get
the extra requirements (wchar, !static).

Fix that with the solution proposed by Thomas.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>