PresetUpdater: Use PID in cache tmp filenames

xs/src/libslic3r/Utils.hpp

@@ -62,6 +62,9 @@ extern std::string timestamp_str();
 // to be placed at the top of Slic3r generated files.
 inline std::string header_slic3r_generated() { return std::string("generated by " SLIC3R_FORK_NAME " " SLIC3R_VERSION " " ) + timestamp_str(); }
 
+// getpid platform wrapper
+extern unsigned get_current_pid();
+
 // Compute the next highest power of 2 of 32-bit v
 // http://graphics.stanford.edu/~seander/bithacks.html
 template<typename T>