Load NSS root certs module

This change is needed to actually do certificate validation, because as it is NSS is trying to load the module which contains all the builtin root certs from the Firefox profile directory where it is usually missing. This way it will load the module from a system library directory.
diff --git a/poppler/SignatureHandler.cc b/poppler/SignatureHandler.cc
index f299b7e..01496e3 100644
--- a/poppler/SignatureHandler.cc
+++ b/poppler/SignatureHandler.cc
@@ -15,6 +15,7 @@
 
 #include "SignatureHandler.h"
 #include "goo/gmem.h"
+#include <nss/secmod.h>
 
 #include <dirent.h>
 #include <Error.h>
@@ -95,6 +96,8 @@
   } else {
     NSS_Init(certDBPath->getCString());
   }
+  //Make sure NSS root certificates module is loaded
+  SECMOD_AddNewModule("Root Certs", "libnssckbi.so", 0, 0);
 
   delete certDBPath;
 }