package/swupdate: add optional systemd dependency

When the swupdate SYSTEMD option is enabled, systemd needs to be built
before swupdate, otherwise the build fails with:

core/notifier.c:27:10: fatal error: systemd/sd-daemon.h: No such file or directory
   27 | #include <systemd/sd-daemon.h>
      |          ^~~~~~~~~~~~~~~~~~~~~

Of course, it remains up to the user to make sure that the systemd
package is enabled when systemd support is enabled in the swupdate
configuration.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit bea0d20c7885df962bdb800189522931a3b129c7)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/swupdate/swupdate.mk

@@ -110,6 +110,10 @@ SWUPDATE_MAKE_ENV += HAVE_MBEDTLS=n
 endif
 endif
 
+ifeq ($(BR2_PACKAGE_SYSTEMD),y)
+SWUPDATE_DEPENDENCIES += systemd
+endif
+
 ifeq ($(BR2_PACKAGE_UBOOT_TOOLS),y)
 SWUPDATE_DEPENDENCIES += uboot-tools
 SWUPDATE_MAKE_ENV += HAVE_LIBUBOOTENV=y