Check if HAVE_STRING_H is defined before using it and fix function signature for ud_opr_is_gpr (bug #41)

I'm not sure where HAVE_STRING_H is supposed to be defined, but I get a build error when I try to build this [as part of WebKit in Xcode](https://bugs.webkit.org/show_bug.cgi?id=117205). This fix checks if it's defined first.

The other fix is renaming `ud_opr_isgpr` to `ud_opr_is_gpr` in extern.h to match udis86.c.