arm64: kernel 5.15: enable TCP_MD5SIG

For BGP TCP MD5 authentication to work, the appropriate support needs to
be enabled in the kernel, so enable it.

Fixes errors from FRR's bgpd like:

    Jul 19 12:29:43 localhost bgpd[954]: [KTTWK-0CPJ7][EC 100663303] sockopt_tcp_signature: setsockopt(22): Protocol not available
    Jul 19 12:29:43 localhost bgpd[954]: [NWGVJ-FEW9F][EC 33554495] Unable to set TCP MD5 option on socket for peer 10.0.1.2 (sock=22): Protocol not available
    Jul 19 12:29:43 localhost bgpd[954]: [KTTWK-0CPJ7][EC 100663303] sockopt_tcp_signature: setsockopt(22): Protocol not available
    Jul 19 12:29:43 localhost bgpd[954]: [NWGVJ-FEW9F][EC 33554495] Unable to set TCP MD5 option on socket for peer 10.0.0.2 (sock=22): Protocol not available

Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>

Author: KanjiMonster

packages/base/any/kernels/5.15-lts/configs/arm64-all/arm64-all.config

@@ -967,7 +967,7 @@ CONFIG_INET_TCP_DIAG=y
 # CONFIG_TCP_CONG_ADVANCED is not set
 CONFIG_TCP_CONG_CUBIC=y
 CONFIG_DEFAULT_TCP_CONG="cubic"
-# CONFIG_TCP_MD5SIG is not set
+CONFIG_TCP_MD5SIG=y
 CONFIG_IPV6=y
 # CONFIG_IPV6_ROUTER_PREF is not set
 # CONFIG_IPV6_OPTIMISTIC_DAD is not set