enable mruby scripting in the nghttpx proxy

This will be needed for (future) handling of TLS certificates.
Apparently, this scripting is the way how it's intended to pass metadata
extracted from clients' TLS certificates to request headers when
proxying requests to the backend server.

A simple buildroot fix is in the `cesnet/2023-09-29` branch.

Change-Id: I56f0481fa24bab2451793c49149cdd50732941c3
diff --git a/configs/czechlight_clearfog_defconfig b/configs/czechlight_clearfog_defconfig
index 20781b2..09151d9 100644
--- a/configs/czechlight_clearfog_defconfig
+++ b/configs/czechlight_clearfog_defconfig
@@ -52,6 +52,7 @@
 BR2_PACKAGE_LIBGPIOD_TOOLS=y
 BR2_PACKAGE_NETOPEER2=y
 BR2_PACKAGE_NETOPEER2_CLI=y
+BR2_PACKAGE_NGHTTP2_MRUBY=y
 BR2_PACKAGE_ELFUTILS=y
 BR2_PACKAGE_LIBUNWIND=y
 BR2_PACKAGE_LINUX_PAM=y