xmlsec: fix linkage, probably after #909

This fixes builds of (some) reverse dependencies, e.g. aqbanking.

(cherry picked from commit e69306c46394f98e8ae53dd21dea3d0495a90780)

pkgs/development/libraries/xmlsec/default.nix

···
   enableParallelBuilding = true;
   doCheck = true;
 
+  # otherwise libxmlsec1-gnutls.so won't find libgcrypt.so, after #909
+  NIX_LDFLAGS = [ "-lgcrypt" ];
+
   postFixup = ''
     wrapProgram "$out/bin/xmlsec1" --prefix LD_LIBRARY_PATH ":" "$out/lib"
   '';