Fix ISO C++ ambiguity compiler warning

Enhance code to pass compiler warning.  Prior to this change the
compiler would complain with the following message:

error: ISO C++ says that these are ambiguous, even though the worst
conversion for the first is better than the worst conversion for the
second: /usr/include/c++/4.5/bits/postypes.h:192:7: note: candidate 1:
std::fpos<_StateT> std::fpos<_StateT>::operator-(std::streamoff) const
[with _StateT = __mbstate_t, std::fpos<_StateT> =
std::fpos<__mbstate_t>, std::streamoff = long int] Utils.cc:443:27:
note: candidate 2: operator-(std::streamoff, long long int) <built-in>

src/Utils.cc

@@ -439,7 +439,7 @@ Glib::ustring Utils::cleanup_cursor( const Glib::ustring & text )
 				break;
 			case '\b':
 				if (out.tellp() > startofline)
-					out.seekp(out.tellp()-1ll);
+					out.seekp(out.tellp() - std::streamsize('\b'));
 				break;
 			default:
 				out.put(ch);