lib/jsonwrt: don't use ctype.h for ASCII chars

lib/jsonwrt: don't use ctype.h for ASCII chars

tolower() does not work "as expected" for tr_TR.UTF-8 (Turkish).
Fortunately, we need to convert only objects and variables names in
JSON output, and this is always old good ASCII.

Anyway, for more details:

	$ cat a.c
	#include <ctype.h>
	#include <stdio.h>
	#include <locale.h>

	int main(void)
	{
		int in, out;

		setlocale(LC_ALL, "");

		in ='I';
		out = tolower(in);

		printf("%1$c [%1$d] --> %2$c [%2$d]\n", in, out);
		return 0;
	}

	$ make a
	cc     a.c   -o a

	$ LANG=en_US.utf8 ./a
	I [73] --> i [105]

	$ LANG=tr_TR.UTF-8 ./a
	I [73] --> I [73]

Fixes: https://github.com/karelzak/util-linux/issues/1302
Signed-off-by: Karel Zak <kzak@redhat.com>