SGPath: mark the str() and utf8Str() methods as 'noexcept'

We'll use this to cleanly declare std::hash<SGPath> as 'noexcept' in the
next commit, which appears to be desirable for std::hash template
specializations, according to:

  http://en.cppreference.com/w/cpp/utility/hash

simgear/misc/sg_path.hxx

@@ -182,8 +182,8 @@ public:
      * Get the path string
      * @return path string
      */
-    std::string str() const { return path; }
-    std::string utf8Str() const { return path; }
+    std::string str() const noexcept { return path; }
+    std::string utf8Str() const noexcept { return path; }
 
     std::string local8BitStr() const;