ixl: quiet -Wunused-function

Direct commit to stable/12.

svn r363876 / git 2bba796c66ede62803ed5da3d83e7efb70c5e9bc MFC'd head
svn r361992 / git b4a7ce0690aedd9763b3b47ee7fcdb421f0434c7. These two
prototypes were brought back, but the definitions were not. The
definitions had been introduced earlier for unrelated work, in svn
r353503 / git ba76aa63573f4aeb8fbea6dc85548660cc6d083b, which has not
been MFC'd.

Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27817