xo: Disable -Wunused-but-set-variable.

Presumably these warnings will be fixed upstream at some point.

contrib/libxo/xo/xo.c:99:9: error: variable 'hflag' set but not used [-Werror,-Wunused-but-set-variable]

int hflag = 0, jflag = 0, tflag = 0,
    ^

contrib/libxo/xo/xo.c:99:20: error: variable 'jflag' set but not used [-Werror,-Wunused-but-set-variable]

int hflag = 0, jflag = 0, tflag = 0,
               ^

contrib/libxo/xo/xo.c:99:31: error: variable 'tflag' set but not used [-Werror,-Wunused-but-set-variable]

int hflag = 0, jflag = 0, tflag = 0,
                          ^

contrib/libxo/xo/xo.c:100:2: error: variable 'zflag' set but not used [-Werror,-Wunused-but-set-variable]

zflag = 0, qflag = 0, star1 = 0, star2 = 0;
^

contrib/libxo/xo/xo.c:100:13: error: variable 'qflag' set but not used [-Werror,-Wunused-but-set-variable]

zflag = 0, qflag = 0, star1 = 0, star2 = 0;
           ^

Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D40668