I looked into that directory, but didn't find the Module.symvers file anywhere.
This file is really needed to build the modules and is only created when the whole kernel is build.
From kbuild documentation:
--- 2.4 Preparing the kernel tree for module build
To make sure the kernel contains the information required to
build external modules the target 'modules_prepare' must be used.
'modules_prepare' exists solely as a simple way to prepare
a kernel source tree for building external modules.
Note: modules_prepare will not build Module.symvers even if
CONFIG_MODVERSIONS is set. Therefore a full kernel build
needs to be executed to make module versioning work.
So, davy_gravy, could you post this file?