networkd: silence bogus gcc warning about %s

networkd: silence bogus gcc warning about %s

Fixes #12931.

In file included from ../src/basic/macro.h:558,
                 from ../src/basic/alloc-util.h:9,
                 from ../src/network/networkd-link.c:7:
../src/network/networkd-link.c: In function ‘link_sysctl_ipv6_enabled’:
../src/basic/log.h:107:9: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  107 |         log_internal_realm(LOG_REALM_PLUS_LEVEL(LOG_REALM, (level)), __VA_ARGS__)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/log-link.h:21:25: note: in expansion of macro ‘log_internal’
   21 |                         log_internal(level, error, _FILE_, __LINE__, __func__, ##__VA_ARGS__); \
      |                         ^~~~~~~~~~~~
../src/shared/log-link.h:33:50: note: in expansion of macro ‘log_link_full’
   33 | #define log_link_warning_errno(link, error, ...) log_link_full(link, LOG_WARNING, error, ##__VA_ARGS__)
      |                                                  ^~~~~~~~~~~~~
../src/network/networkd-link.c:83:24: note: in expansion of macro ‘log_link_warning_errno’
   83 |                 return log_link_warning_errno(link, r,
      |                        ^~~~~~~~~~~~~~~~~~~~~~
../src/network/networkd-link.c:84:77: note: format string is defined here
   84 |                                               "Failed to read net.ipv6.conf.%s.disable_ipv6 sysctl property: %m",
      |                                                                             ^~
cc1: some warnings being treated as errors