check: fix werror build with clang

check: fix werror build with clang

Silence -Wformat-nonliteral warnings from the internal copy of libcheck

../subprojects/gstreamer/libs/gst/check/libcheck/check.c:379:29: warning: format string is not a string literal [-Wformat-nonliteral]
    vsnprintf (buf, BUFSIZ, msg, ap);
                            ^~~
../subprojects/gstreamer/libs/gst/check/libcheck/check_error.c:48:21: warning: format string is not a string literal [-Wformat-nonliteral]
  vfprintf (stderr, fmt, args);
                    ^~~
../subprojects/gstreamer/libs/gst/check/libcheck/check_str.c:92:29: warning: format string is not a string literal [-Wformat-nonliteral]
    n = vsnprintf (p, size, fmt, ap);
                            ^~~