make install gives depmod: Warning: ... needs unknown symbol about netmap HOT 2 OPEN

I've made a little progress but still can't solve the problem:

For the first few steps I get output like

make[1]: Entering directory '/usr/src/linux-headers-5.10.0-20-amd64'
  INSTALL /home/alex/OSVirt/netmap/netmap.ko
  DEPMOD  5.10.0-20-amd64
Warning: modules_install: missing 'System.map' file. Skipping depmod.

indicating that there is no System.map file. As far as I understand the depmod(8) manpage, this is normal when using the active kernel (and map files are only created for other kernels).
The script then calls /sbin/depmod -e -F /boot/System.map-5.10.0-20-amd64 -a 5.10.0-20-amd64 with the map file under /boot with the dummy contents:

ffffffffffffffff B The real System.map is in the linux-image-<version>-dbg package

However, as stated above, I did not install the full kernel sources, hence linux-image-xx does not exist. This explains the warnings, as it cannot find any symbols in the map file, -eF will report every symbol as missing.
I do not do what exactly to do about it and why it ultimately fails though.

from netmap.

Hi @AlexTUM, the latest master should fix the installation error. You can ignore the warnings.

from netmap.