boost: fix patches for 1.87.0 with structuredAttrs

This properly flattens the patches array again -
otherwise all patches are dropped when structuredAttrs are enabled.

The build will finish, but downstream users of the library will
not be able to link to it properly (e.g. folly).

pkgs/development/libraries/boost/generic.nix

@@ -226,7 +226,7 @@ stdenv.mkDerivation {
         hash = "sha256-0IHK55JSujYcwEVOuLkwOa/iPEkdAKQlwVWR42p/X2U=";
       })
     ]
-    ++ lib.optional (version == "1.87.0") [
+    ++ lib.optionals (version == "1.87.0") [
       # Fix operator<< for shared_ptr and intrusive_ptr
       # https://github.com/boostorg/smart_ptr/issues/115
       (fetchpatch {
@@ -402,4 +402,6 @@ stdenv.mkDerivation {
     "dev"
   ];
   setOutputFlags = false;
+
+  __structuredAttrs = true;
 }