shell: fix compile error due to -Werror

Remove -Werror to prevent following error:

  ../../lib/stdio.c: In function '_PrintHex':
  ../../lib/stdio.c:174:6: error: variable 'cnt' set but not used [-Werror=unused-but-set-variable]
  ../../lib/stdio.c:170:22: error: variable 'flagl' set but not used [-Werror=unused-but-set-variable]
  ../../lib/stdio.c: In function 'strtoul':
  ../../lib/stdio.c:414:16: error: variable 'strtoul_err' set but not used [-Werror=unused-but-set-variable]
  cc1: all warnings being treated as errors
  make[1]: *** [../../lib/stdio.o] Error 1

Signed-off-by: coldnew <coldnew.tw@gmail.com>