tracetool: report error on foo() instead of foo(void)

C functions with no arguments must be declared foo(void) instead of
foo().  The tracetool argument list parser has never accepted an empty
argument list.  This patch adds a clear error message for this error
case.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180110202553.31889-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>