make: garbage in, garbage out for live mount

(cherry picked from commit 531e3036)

Makefile

@@ -26,6 +26,7 @@ umount: force
 	@if [ -f ${WRKDIR}/.mount_done ]; then \
 	    echo -n "Disabling core.git live mount..."; \
 	    umount -f "<above>:${.CURDIR}/src"; \
+	    rm ${.CURDIR}/src/opnsense/version/opnsense; \
 	    rm ${WRKDIR}/.mount_done; \
 	    echo "done"; \
 	    service configd restart; \