docs: Document CONFIG_PERF_EVENTS requirement

This is needed for Hubble events and connection tracking garbage
collection. Furthermore, a user reported that without this option,
Cilium crashes on startup with the following message:

    level=fatal msg="Cannot initialise BPF perf ring buffer sockets"
    error="failed to create perf ring for CPU 0:
           can't create perf event:
	   function not implemented"

Signed-off-by: Joe Stringer <[email protected]>

Documentation/operations/system_requirements.rst

@@ -171,6 +171,7 @@ linked, either choice is valid.
         CONFIG_CRYPTO_USER_API_HASH=y
         CONFIG_CGROUPS=y
         CONFIG_CGROUP_BPF=y
+        CONFIG_PERF_EVENTS=y
 
 
 Requirements for Iptables-based Masquerading