build-sys: Fix daemon linking when building with systemd-daemon support

Seems that after commit 467b4b9be systemd usage has been added into
src/daemon/main.c but there is no link for the corresponding
library in the final pulseaudio binary.

This might be missed in some build systemd due to overlinking,
but it's correct to add this in here explicitly

Signed-off-by: Colin Guthrie <colin@mageia.org>