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
