Makefle: force char to be signed

On an armv7l build, we see errors like

 warning: rgw/rgw_common.cc:626:16: comparison is always false due to limited range of data type [-Wtype-limits]

from code

      char c1 = hex_to_num(*src++);
...
      if (c1 < 0)

Force char to be signed (regardless of any weird architecture's default)
to avoid risk of this leading to misbehavior.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 769a16d6674122f3b537f03e17514ad974bf2a2f)