Fix arm builds of updated libedit

[dim]

Summary

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)
                                           ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~
2 errors generated.
*** [terminal.o] Error code 1

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

[bapt]

Note that the same patch was provided by @naito.yuichiro_gmail.com in D21584 (and he contacted upstream for it to be committed in.) Please mention him also in the commit log