0000851: toolchain/kernel-headers: Use LINUX_HEADERS_VERSION instead of

LINUX_VERSION.
 
Use LINUX_HEADERS_VERSION instead of LINUX_VERSION as that variable is used
for the actual kernel version used which might be different than the headers
version.

E.G. LINUX_VERSION is set in the target/device/* files. If this version
doesn't match the selected kernel headers version the patches won't be applied.