Fix cast warning in Unicode function

Fixes #339. Fixes a sign conversion warning in a function compiled by
-DCXXOPTS_USE_UNICODE_HELP=1.

CHANGELOG.md

@@ -8,6 +8,7 @@ options. The project adheres to semantic versioning.
 ### Bug Fixes
 
 * Fixed version number in header.
+* Fixed cast warning in Unicode function.
 
 ## 3.1
 

include/cxxopts.hpp

@@ -230,10 +230,10 @@ stringAppend(String& s, Iterator begin, Iterator end)
 }
 
 inline
-std::size_t
+size_t
 stringLength(const String& s)
 {
-  return s.length();
+  return static_cast<size_t>(s.length());
 }
 
 inline