It looks like the OSMC headers are still broken.
I see you figured out the need to add the “build” symbolic link. Well done.
There is a workaround detailed here. You’ll also need to download the kernel source.
Edit:
Just tried the workaround. Success!
make ARCH=arm CROSS_COMPILE= -C /lib/modules/4.14.78-4-osmc/build M=/home/osmc/rtl8822bu modules
make[1]: Entering directory '/usr/src/rbp2-headers-4.14.78-4-osmc'
<snip>
Building modules, stage 2.
MODPOST 1 modules
LD [M] /home/osmc/rtl8822bu/8822bu.ko
make[1]: Leaving directory '/usr/src/rbp2-headers-4.14.78-4-osmc'