bib: Add DOI for Nidhugg-parallel

Signed-off-by: Michalis Kokologiannakis <mixaskok@gmail.com>

biblio.bib

@@ -2589,6 +2589,7 @@
   address	= {Cham},
   pages		= {377--393},
   isbn		= {978-3-030-59152-6},
+  doi		= {10.1007/978-3-030-59152-6_21},
   ids		= {nidhugg-parallel,parallel-nidhugg}
 }