msmtp: Fix GSSAPI support

We already have gsasl as a build input, but it was not being used
because we were missing the --with-libgsasl configure flag.

Signed-off-by: Anders Kaseorg <[e-mail]>

pkgs/applications/networking/msmtp/default.nix

···
 
   nativeBuildInputs = [ autoreconfHook pkg-config texinfo ];
 
-  configureFlags =
-    [ "--sysconfdir=/etc" ] ++ lib.optional stdenv.isDarwin [ "--with-macosx-keyring" ];
+  configureFlags = [ "--sysconfdir=/etc" "--with-libgsasl" ]
+    ++ lib.optional stdenv.isDarwin [ "--with-macosx-keyring" ];
 
   postInstall = ''
     install -d $out/share/doc/${pname}/scripts