the license of the v4l2loopback.h header is GPL-2.0+ WITH Linux-syscall-note: https://github.com/umlaeute/v4l2loopback/blob/5bb9bed4e2a34761dc5dc188faec7315ae3930fe/v4l2loopback.h#L1

The Linux-syscall-note exception can be found at https://spdx.org/licenses/Linux-syscall-note.html
the relevant part is:

NOTE! This copyright does not cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does not fall under the heading of "derived work".

as such, any program using the header in order to issue syscalls, is not covered by the GPL-2.0+

see also https://opensource.stackexchange.com/q/6647