developer_guides: llext_modules.rst: add reference for Intel firmware look-up paths

add the missing label definition for intel_firmware_path to resolve
the docs build error

Signed-off-by: Christopher Turner <christopher.g.turner@intel.com>

Author: cgturner1

developer_guides/firmware/llext_modules.rst

@@ -76,6 +76,7 @@ modules don't have to use "tristate" in their Kconfig entries.
 
 Installation
 ************
+.. _intel_firmware_paths:
 
 As specified in
 :ref:`Firmware look-up paths per Intel platform <intel_firmware_paths>`