Add .vendor to LOCAL_REQUIRED_MODULES for vendor

This commit adds ".vendor" suffix to the modules specified in
LOCAL_REQUIRED_MODULES when BOARD_VNDK_VERSION is not empty and it is
building the vendor variant.

Test: Create a vendor module with LOCAL_REQUIRED_MODULES to a
vendor_available shared library.
Change-Id: If448f682572157d0687cda3a72f9bab0f34d7f0d