configs/beaglev_defconfig: update linux-headers to 5.13.

The beaglev kernel is based on the 5.13 branch, update
the expected linux-headers version to 5.13.

This has been wrong ever since the bump of the kernel version in commit
9a1bd7cc1ce512672a5d76baa86c449e36137052: the headers were bumped to
5.12 instead of 5.13.

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1525740895



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>