Fix issues when compiling dtc with Cygwin

Original author: rlong

Currently, DTC is failing to build due to this check in checks.c. For some reason the argument into isxdigit is coming in as a char. I've brought it up to the maintainer, but he sees no reason to change it. Therefore, a patch is needed to get it to build.

1183 if (unitname[0] == '0' && isxdigit(unitname[1])) 1184 FAIL(c, dti, node, "unit name should not have leading 0s"); 1185 }