BUILD: makefile: fix build of IPv6 header on aix51

ip6_hdr is called ip6hdr there and is only defined when STEVENS_API is
defined.
diff --git a/Makefile b/Makefile
index 26dfcb4..b3a8e7a 100644
--- a/Makefile
+++ b/Makefile
@@ -382,7 +382,7 @@
 ifeq ($(TARGET),aix51)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_LIBCRYPT USE_OBSOLETE_LINKER)
-  TARGET_CFLAGS   = -Dss_family=__ss_family
+  TARGET_CFLAGS   = -Dss_family=__ss_family -Dip6_hdr=ip6hdr -DSTEVENS_API
   DEBUG_CFLAGS    =
 endif