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