Hack together a fix for the public headers

Please fix this.

src/libmain/package.nix

@@ -6,6 +6,7 @@
 
   nix-util,
   nix-store,
+  nix-expr,
 
   # Configuration Options
 
@@ -33,6 +34,10 @@ mkMesonLibrary (finalAttrs: {
   ];
 
   propagatedBuildInputs = [
+    # FIXME: This is only here for the NIX_USE_BOEHMGC macro dependency
+    #        Removing nix-expr will make the build more concurrent and is
+    #        architecturally nice, perhaps.
+    nix-expr
     nix-util
     nix-store
     openssl