package/systemd: Force enable blkid

Since systemd select BR2_PACKAGE_UTIL_LINUX_LIBBLKID, we can safely
force this option.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/systemd/systemd.mk

@@ -27,6 +27,7 @@ endif
 
 SYSTEMD_CONF_OPTS += \
 	--with-rootprefix= \
+	--enable-blkid \
 	--enable-static=no \
 	--disable-manpages \
 	--disable-selinux \