Bug#1023607: 50-depmod.install fails if kmod is not installed

[Max Kellermann]

Package: systemd
Version: 252-2

My computer has custom-built kernels without module support, and
therefore I did not install kmod.  When installing a kernel package, I get:

 /usr/lib/kernel/install.d/50-depmod.install: 28: exec: depmod: not found

This script comes with the systemd package, but systemd does not
depend on kmod, and doing so would be the wrong solution; the proper
solution is to check whether "depmod" is installed in the script.