lib.nng: fix mbedtlsSupport override

previously overriding the mbedtlsSupport flag
would result in a configure error because the build would still search for
the necessarry mbedtls libraries

pkgs/development/libraries/nng/default.nix

@@ -16,8 +16,8 @@ stdenv.mkDerivation rec {
 
   buildInputs = lib.optionals mbedtlsSupport [ mbedtls ];
 
-  cmakeFlags = [ "-G Ninja" "-DNNG_ENABLE_TLS=ON" ]
-    ++ lib.optionals mbedtlsSupport [ "-DMBEDTLS_ROOT_DIR=${mbedtls}" ];
+  cmakeFlags = [ "-G Ninja" ]
+    ++ lib.optionals mbedtlsSupport [ "-DMBEDTLS_ROOT_DIR=${mbedtls}" "-DNNG_ENABLE_TLS=ON" ];
 
   meta = with lib; {
     homepage = "https://nng.nanomsg.org/";