systemd: recommend libelf, libdw for elfutils flag

systemd: recommend libelf, libdw for elfutils flag

when elfutils is enabled, the elf reading code of systemd will dlopen
libelf and libdw. these dependencies are not automatically detected and
will not work at runtime without them installed. add them to RRECOMMENDS
when elfutils is turned on in PACKAGECONFIG.

fixes https://bugzilla.yoctoproject.org/show_bug.cgi?id=15379

Signed-off-by: Nick Owens <nick.owens@eero.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>