If you even define KBUILD_EXTRA_SYMBOLS in Makefile it will not be
expanded into command line argument for modpost. Patch in a

Note: this bug was reported and kernel bugzilla:
http://bugzilla.kernel.org/show_bug.cgi?id=11567
and patch was included in -mm tree.

The patch was generated against 2.6.26 kernel but I suppose it should
work applies to the linux/kernel/git/torvalds/linux-2.6.git too.

-- 
Peter.