build: prevent redefinition of _FORTIFY_SOURCE in gentoo

Add a -U_FORTIFY_SOURCE in HARDENING=1 [1] to prevent [2], e.g. as in:

x86_64-pc-linux-gnu-gcc -march=native -O2 -pipe -std=gnu99 -pipe -fPIE
  -pie -Wl,-z,relro,-z,now -fstack-protector-all -Wstack-protector
  --param=ssp-buffer-size=4 -ftrapv -D_FORTIFY_SOURCE=2 -fexceptions
  -fomit-frame-pointer -fno-strict-aliasing -fasynchronous-unwind-tables
  -fno-delete-null-pointer-checks -D_REENTRANT -D_LARGEFILE_SOURCE
  -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -DVERSION_STRING=\""0.5.8-rc2"""\"
  -DVERSION_LONG=\""0.5.8-rc2"" (Ziggomatic)"\" -DPREFIX_STRING=\"\"
  -Wall -I. -I/usr/include/libnl3 -I/usr/include/libnl3
  -DHAVE_DISSECTOR_PROTOS -DNEED_TCPDUMP_LIKE_FILTER -I/usr/include/libnl3
  -I/usr/include/libnl3 -DHAVE_DISSECTOR_PROTOS -DNEED_TCPDUMP_LIKE_FILTER
  -o netsniff-ng/proto_igmp.o -c proto_igmp.c
:0:0: warning: "_FORTIFY_SOURCE" redefined [enabled by default]
proto_igmp.c:1:0: note: this is the location of the previous definition

[1] http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/net-analyzer/netsniff-ng/files/netsniff-ng-0.5.8-fortify_source.patch?revision=1.1&view=markup
[2] http://b-4.xmw.de/var/log/portage/build/net-analyzer/netsniff-ng-0.5.8_rc2:20130805-070734.log

Reported-by: Michael Weber <michael@xmw.de>
Suggested-by: Michael Weber <michael@xmw.de>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>