add linux shared library.

test/kdbx4_test.dart

@@ -42,7 +42,9 @@ typedef Argon2Hash = Pointer<Utf8> Function(
 class Argon2Test implements Argon2 {
   Argon2Test() {
 //    final argon2lib = DynamicLibrary.open('libargon2.1.dylib');
-    final argon2lib = DynamicLibrary.open('libargon2_ffi.dylib');
+    final argon2lib = Platform.isMacOS
+        ? DynamicLibrary.open('libargon2_ffi.dylib')
+        : DynamicLibrary.open('libargon2_ffi.so');
     _argon2hash = argon2lib
         .lookup<NativeFunction<Argon2HashNative>>('hp_argon2_hash')
         .asFunction();