support CONFIG_MODVERSIONS=y

When CONFIG_MODVERSIONS is enabled, loading of the patch module fails
with "no symbol version for kpatch_register".  When building the patch
module, we need to point it to the core module's Module.symvers file.

This also works when CONFIG_MODVERSIONS is disabled, since
Module.symvers is created regardless.
Merge pull request #94 from jpoimboe/module-symvers

support CONFIG_MODVERSIONS=y