pinentry: fix cross

When cross-compiling, pinentry's configury needs extra hints about
where to find libassuan and libgpg-error.  Let's provide them.

pkgs/tools/security/pinentry/default.nix

@@ -62,6 +62,8 @@ pinentryMkDerivation rec {
   ];
 
   configureFlags = [
+    "--with-libgpg-error-prefix=${libgpg-error.dev}"
+    "--with-libassuan-prefix=${libassuan.dev}"
     (lib.enableFeature withLibsecret "libsecret")
   ] ++ (map enableFeaturePinentry (lib.attrNames flavorInfo));