Specify signed char
Specify signed char

x86 architecture default to signed characters.  Other architectures
default to unsigned characters.  When building on non-x86 arch we
get the following error

./json.hpp:22700:42:   required from here
./json.hpp:8494:24: error: comparison is always true due to limited
                           range of data type [-Werror=type-limits]
 8494 |             if ('\x00' <= c and c <= '\x1F')
      |                 ~~~~~~~^~~~

Signed-off-by: Tony Asleson <1ef2d381aa56d512126ddaacc29e68136e4fdf55@redhat.com>

tasleson/libstoragemgmt,libstorage/libstoragemgmt,libstorage/libstoragemgmt,cathay4t/libstoragemgmt,cathay4t/libstoragemgmt,libstorage/libstoragemgmt,libstorage/libstoragemgmt,tasleson/libstoragemgmt,tasleson/libstoragemgmt,tasleson/libstoragemgmt,libstorage/libstoragemgmt,cathay4t/libstoragemgmt,cathay4t/libstoragemgmt,cathay4t/libstoragemgmt,tasleson/libstoragemgmt