Fix build issue that occurs on gcc debian 6.3.0-18

  CC      lease.o
In file included from /usr/include/arpa/inet.h:22:0,
                 from lease.c:3:
lease.c: In function 'new_lease':
lease.c:93:30: error: 'const struct in6_addr' has no member named 's6_addr32'
  bool wants_ipv6 = !ipv6 || !IN6_IS_ADDR_UNSPECIFIED(ipv6);
                              ^
lease.c:93:30: error: 'const struct in6_addr' has no member named 's6_addr32'
  bool wants_ipv6 = !ipv6 || !IN6_IS_ADDR_UNSPECIFIED(ipv6);
                              ^
lease.c:93:30: error: 'const struct in6_addr' has no member named 's6_addr32'
  bool wants_ipv6 = !ipv6 || !IN6_IS_ADDR_UNSPECIFIED(ipv6);
                              ^
lease.c:93:30: error: 'const struct in6_addr' has no member named 's6_addr32'
  bool wants_ipv6 = !ipv6 || !IN6_IS_ADDR_UNSPECIFIED(ipv6);
                              ^
lease.c: In function 'leases_refresh':
lease.c:213:10: error: 'const struct in6_addr' has no member named 's6_addr32'
     if (!IN6_IS_ADDR_UNSPECIFIED(ipv6))
          ^
lease.c:213:10: error: 'const struct in6_addr' has no member named 's6_addr32'
     if (!IN6_IS_ADDR_UNSPECIFIED(ipv6))
          ^
lease.c:213:10: error: 'const struct in6_addr' has no member named 's6_addr32'
     if (!IN6_IS_ADDR_UNSPECIFIED(ipv6))
          ^
lease.c:213:10: error: 'const struct in6_addr' has no member named 's6_addr32'
     if (!IN6_IS_ADDR_UNSPECIFIED(ipv6))
          ^
<builtin>: recipe for target 'lease.o' failed
make: *** [lease.o] Error 1

1 files changed, 5 insertions, 1 deletions

diff --git a/lease.c b/lease.c
index 73028aa..eac5a15 100644
--- a/lease.c
+++ b/lease.c
@@ -1,4 +1,8 @@
-#define _POSIX_C_SOURCE 200809L
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2019 WireGuard LLC. All Rights Reserved.
+ */
+#define _GNU_SOURCE
 
 #include <arpa/inet.h>
 #include <inttypes.h>