virkeyfile: fix compilation error with clang

Clang complains about condition being always true:

src/util/virkeyfile.c:113:23: error: result of comparison of constant 128 with expression of type 'const char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
    while (!IS_EOF && IS_ASCII(CUR) && CUR != ']')
                      ^~~~~~~~~~~~~
src/util/virkeyfile.c:80:26: note: expanded from macro 'IS_ASCII'
                     ~~~ ^ ~~~
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/util/virkeyfile.c

@@ -77,7 +77,7 @@ struct _virKeyFileParserCtxt {
 #define IS_EOF (ctxt->cur >= ctxt->end)
 #define IS_EOL(c) (((c) == '\n') || ((c) == '\r'))
 #define IS_BLANK(c) (((c) == ' ') || ((c) == '\t'))
-#define IS_ASCII(c) ((c) < 128)
+#define IS_ASCII(c) (((unsigned char) (c)) < 128)
 #define CUR (*ctxt->cur)
 #define NEXT if (!IS_EOF) ctxt->cur++;