lorax-templates-qubes: don't fail when no extra kernel is selected

It should be possible to build the ISO with just one kernel from
repository, instead of locally built.

lorax-templates-qubes/templates/x86.tmpl

@@ -37,7 +37,7 @@ install boot/memtest* ${BOOTDIR}/memtest
 
 mkdir ${EXTRAKERNELS}
 # bypass "chroot" enforced by Lorax parser
-install ../../../../yum/qubes-dom0/rpm/kernel-qubes-vm-[0-9]*rpm ${EXTRAKERNELS}
+-install ../../../../yum/qubes-dom0/rpm/kernel-qubes-vm-[0-9]*rpm ${EXTRAKERNELS}
 
 ## install kernels
 mkdir ${KERNELDIR}