package/systemd: fix build with fallback hostname

Commit aebabb2780da4c5f85fec00b56f4690feb646073 used an automoake
configuration option. Replace it with the meson version.

Fixes #10716

Signed-off-by: Stefan Becker <chemobejk@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/systemd/systemd.mk

@@ -319,7 +319,7 @@ endif
 
 SYSTEMD_FALLBACK_HOSTNAME = $(call qstrip,$(BR2_TARGET_GENERIC_HOSTNAME))
 ifneq ($(SYSTEMD_FALLBACK_HOSTNAME),)
-SYSTEMD_CONF_OPTS += --with-fallback-hostname=$(SYSTEMD_FALLBACK_HOSTNAME)
+SYSTEMD_CONF_OPTS += -Dfallback-hostname=$(SYSTEMD_FALLBACK_HOSTNAME)
 endif
 
 define SYSTEMD_INSTALL_INIT_HOOK