doc: add libbpf to optional requirements

doc/requirements.rst

@@ -55,11 +55,6 @@ Operating system
 Knot DNS itself is written in a portable way and can be compiled
 and run on most UNIX-like systems, such as Linux, \*BSD, and macOS.
 
-The :ref:`XDP funcionality <Mode XDP>` and :doc:`kxdpgun <man_kxdpgun>`
-tool are only supported on Linux operating systems. See the chapter
-:ref:`Mode XDP <Mode XDP_pre-requisites>` for software and hardware
-recommendations.
-
 Required libraries
 ==================
 
@@ -103,3 +98,9 @@ DNS-over-HTTPS (DoH) support in :doc:`kdig<man_kdig>`:
 
 * libnghttp2
 
+The :ref:`XDP funcionality <Mode XDP>` and :doc:`kxdpgun <man_kxdpgun>`
+tool. These are only supported on Linux operating systems. See the chapter
+:ref:`Mode XDP <Mode XDP_pre-requisites>` for software and hardware
+recommendations.
+
+* libbpf >= 0.0.6