[Problem Solved]
The kernel header files are indeed on the system, but are not found by a check in the Makefile, causing the process to stop. By disabling this test in the Makefile (or by removing “exit”) make can continue and the RTC modules are created. The hwclock program is not included by default in Trixie (Debian 13), so you need to install the util-linux-extra package. As a result, the hardware clock (RTC) now works.