keep-prefix keeps relative pathing (#7394)

include/flatbuffers/util.h

@@ -448,6 +448,9 @@ std::string StripPath(const std::string &filepath);
 // Strip the last component of the path + separator.
 std::string StripFileName(const std::string &filepath);
 
+std::string StripPrefix(const std::string &filepath,
+                        const std::string &prefix_to_remove);
+
 // Concatenates a path with a filename, regardless of whether the path
 // ends in a separator or not.
 std::string ConCatPathFileName(const std::string &path,