Support for genfs labels in build_target_files

system/etc/selinux/plat_sepolicy_genfs_{ver}.cil must be included when
generating a precompiled sepolicy binary.

Bug: 409353466
Test: manually run merge_target_files and check the secilc command
Change-Id: Id27332fca9e213bbd8e66555a1dc5c023af747b7