pppd: Provide error() implementation in pppoe-discovery

pppd: Provide error() implementation in pppoe-discovery

The pppoe-discovery program calls error() from the CHECK_ROOM macro
defined in pppoe.h. Since pppoe-discovery is a standalone program not
linked with the rest of pppd, the only way this could build is by
linking to glibc's proprietary error(3) function instead of the function
of the same name (but with different arguments) defined in pppd/utils.c.

So with glibc this builds, but will probably crash when the assertion is
triggered. As the assertion is unlikely to fail, nobody has noticed.

The build however fails with musl libc or uClibc since they don't
provide the doppelganger.

Signed-off-by: Stefan Nickl <Stefan.Nickl@gmail.com>