incfs: fix uninitialized values

Clang's static analyzer is flagging uninitialized struct fields that
we're passing around. In particular, it's not happy that we pass this
partially-initialized struct to a pure-virtual method. I tend to agree
that this may cause issues.

Assuming zero-init gives us reasonable values for all of:

IncFsNewFileParams{}.verification.rootHash
IncFsNewFileParams{}.verification.additionalData
IncFsNewFileParams{}.verification.signature

Bug: None
Test: TreeHugger
Change-Id: I61e556cd8c0e68cdaebd50b0a7be5d5e0a4fd8ff