syz-manager: use compile units for file coverage filter
syz-manager: use compile units for file coverage filter

Don't assume that symbols are always available
and use compile units for file filter instead.
gvisor coverage won't expose symbols, but will have units.

google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller