libalias: Fix compile time warning about unused functions

Compiling libalias results in warnings about unused functions.
Those warnings are caused by clang's heuristic to consider an inline
function as in use, iff the declaration is in a *.c file.
Declarations in *.h files do not emit those warnings.

Hence the declarations must be moved to an extra *.h file.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30844