bpf-headers: Fix download URL with kernel 6.1

bpf-headers: Fix download URL with kernel 6.1

This fixes the download of the kernel 5.15 for the bpf-headers when
kernel 6.1 is build for the target.

Even if kernel 6.1 was selected for the target we still use kernel 5.15
for the bpf-headers. The download script tried to download the 5.15
kernel from the 6.x directory on kernel,org and this failed. Define
PKG_SOURCE_URL based on PKG_PATCHVER and not KERNEL_BASE like done in
kernel.mk.

Without this change it tries to download the kernel from this URL:
ttps://cdn.kernel.org/pub/linux/kernel/v6.x/linux-5.15.129.tar.xz

Fixes: #13190
Fixes: #13671
Fixes: #13814
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>