qjson.h: Remove spurious GCC_FMT_ATTR markup from qobject_from_json() declaration

The function qobject_from_json() doesn't actually allow its
argument to be a format string -- it passes a NULL va_list*
to qobject_from_jsonv(), and the parser code will then never
actually interpret %-escape sequences (it tests whether the
va_list pointer is NULL and will stop with a parse error).

The spurious attribute markup causes clang warnings in some
of the test cases where we programmatically construct JSON
to feed to qobject_from_json():

 tests/test-qmp-input-visitor.c:76:35: warning: format string is not a
 string literal (potentially insecure) [-Wformat-security]
    data->obj = qobject_from_json(json_string);
                                  ^~~~~~~~~~~

Remove the incorrect attribute.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

include/qapi/qmp/qjson.h

@@ -19,7 +19,7 @@
 #include "qapi/qmp/qobject.h"
 #include "qapi/qmp/qstring.h"
 
-QObject *qobject_from_json(const char *string) GCC_FMT_ATTR(1, 0);
+QObject *qobject_from_json(const char *string);
 QObject *qobject_from_jsonf(const char *string, ...) GCC_FMT_ATTR(1, 2);
 QObject *qobject_from_jsonv(const char *string, va_list *ap) GCC_FMT_ATTR(1, 0);