systemd: disable LTO for now

Enabling LTO causes the MIPS build to fail with a bunch of duplicate
symbols from the compat libraries:

  /tmp/ccPreUR5.ltrans0.ltrans.o: In function `sd_booted':
  ccPreUR5.ltrans0.o:(.text+0xe48): multiple definition of `sd_booted'
  libsystemd_internal_la-sd-daemon.o (symbol from plugin):(.text+0x0): first defined here
  /tmp/ccPreUR5.ltrans0.ltrans.o: In function `sd_notifyf':
  ccPreUR5.ltrans0.o:(.text+0xea8): multiple definition of `sd_notifyf'
  libsystemd_internal_la-sd-daemon.o (symbol from plugin):(.text+0x0): first defined here
  ...

Needs further investigation.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>