Fix --with-kernel for libnfnetlink

[Patrick McHardy]

Sven-Haegar Koch wrote:
>> + if test "$with_kernel" = ""; then
>> +   KERNEL="`uname -r`"
>> + else
>> +   KERNEL="$with_kernel"
>> + fi
> 
> 
> Shouldn't --with-kernel let me specify a kernel source directory,
> instead of a kernel version and then using some heuristicts to find the
> source directory? (The heuristic for the default using
> /lib/modules/`uname -r`/build is fine)

I agree, if someone sends me a patch to change it I'll apply it.