Fix arm and aarch64 builds of libedit after r352275

Fix arm and aarch64 builds of libedit after r352275

On arm and arm64, where chars are unsigned by default, buildworld dies
with:

--- terminal.o ---
/usr/src/contrib/libedit/terminal.c:569:41: error: comparison of
integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka
'unsigned int') [-Werror,-Wsign-compare]
                                     el->el_cursor.v][where & 0370] !=
                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/usr/src/contrib/libedit/terminal.c:659:28: error: comparison of
integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka
'unsigned int') [-Werror,-Wsign-compare]
                                     [el->el_cursor.h] == MB_FILL_CHAR)
                                     ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~

Fix this by making MB_FILL_CHAR a wint_t, so no casting is needed.

Note that in https://reviews.freebsd.org/D21584 this was also proposed
by Yuichiro Naito <naito.yuichiro_gmail.com>.

Reviewed by:	bapt
Subscribers:	naito.yuichiro_gmail.com, ml_vishwin.info
MFC after:	3 weeks
X-MFC-With:	r352275
Differential Revision: https://reviews.freebsd.org/D21657