PR #626: make a couple of private functions static

Author: Numpsy

src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs

@@ -243,7 +243,7 @@ public override void Finish()
 
 		#region Support Routines
 
-		private string CleanFilename(string path)
+		private static string CleanFilename(string path)
 			=> path.Substring(path.LastIndexOf('/') + 1);
 
 		private void WriteHeader()

src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs

@@ -2485,7 +2485,7 @@ private void CopyBytes(ZipUpdate update, Stream destination, Stream source,
 		/// <param name="update">The update to get the size for.</param>
 		/// <param name="includingSignature">Whether to include the signature size</param>
 		/// <returns>The descriptor size, zero if there isn't one.</returns>
-		private int GetDescriptorSize(ZipUpdate update, bool includingSignature)
+		private static int GetDescriptorSize(ZipUpdate update, bool includingSignature)
 		{
 			if (!((GeneralBitFlags)update.Entry.Flags).HasFlag(GeneralBitFlags.Descriptor)) 
 				return 0;