Eliminate some GCC warnings

Eliminate some GCC warnings

* src/doprnt.c (exprintf, evxprintf):
* src/lisp.h (exprintf, evxprintf): Don't use a pointer-to-const type
for the `nonheapbuf` argument: although it is never dereferenced, GCC
will warn when passing a pointer to uninitialised memory otherwise.
* src/fns.c (sort_vector_copy, realize_face, realize_gui_face)
(realize_tty_face): Use the same signatures in the prototypes as in
the actual function definitions.