kbuild: disable modpost warnings for linkonce sections

Disable modpost warnings for linkonce sections

My build gives lots of warnings like

WARNING: sound/core/snd.o (.gnu.linkonce.wi.mpspec_def.h.30779716): unexpected section name.
The (.[number]+) following section name are ld generated and not expected.
Did you forget to use "ax"/"aw" in a .S file?
Note that for example <linux/init.h> contains
section definitions for use in .S files.

But for .linkonce. duplicated sections are actually ok and expected.
So just disable the warning for this case.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>