volatile-binds: use PN to replace hardcode name

Otherwise it doesn't work since SYSTEMD_SERVICE_volatile-binds is not defined
when multilib.

(From OE-Core rev: ccdc709655470af942afccda879e6f8484bebc5e)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/volatile-binds/volatile-binds.bb

@@ -67,7 +67,7 @@ do_install () {
     install -m 0755 mount-copybind ${D}${base_sbindir}/
 
     install -d ${D}${systemd_unitdir}/system
-    for service in ${SYSTEMD_SERVICE_volatile-binds}; do
+    for service in ${SYSTEMD_SERVICE_${PN}}; do
         install -m 0644 $service ${D}${systemd_unitdir}/system/
     done