cmFilePathChecksum: Make methods const useable

Source/cmFilePathChecksum.h

@@ -47,13 +47,13 @@ public:
   /* @brief Calculates the path checksum for the parent directory of a file
    *
    */
-  std::string get(const std::string& filePath);
+  std::string get(const std::string& filePath) const;
 
   /* @brief Same as get() but returns only the first length characters
    *
    */
   std::string getPart(const std::string& filePath,
-                      size_t length = partLengthDefault);
+                      size_t length = partLengthDefault) const;
 
 private:
   /// Size of the parent directory list