Fix problems compiling on power8 due to char defaulting to unsigned

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

Author: ghalliday

src/uuids.cpp

@@ -121,7 +121,7 @@ CassError cass_uuid_from_string_n(const char* str,
   const char* pos = str;
   char buf[16];
 
-  static const char hex_to_half_byte[256] = {
+  static const signed char hex_to_half_byte[256] = {
     -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,
     -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,
     -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,