package/iptables: nfsynproxy not available on musl

utils/nfsynproxy.c needs several TCP_* constants which are not yet
provided by musl, they were added Jan 17, 2016 to musl:
http://git.musl-libc.org/cgit/musl/commit/?id=53f41fb568ae43034c9876cc9bd3961fd6d13671

Until we support a musl version containing the constants we disable
the option for building nfsynproxy on musl.

Fixes
http://autobuild.buildroot.net/results/8ee/8ee299d8697bf488ba94f60785e81f274b2128fd/
http://autobuild.buildroot.net/results/f1a/f1ab3fe59d0dd7dd7b9a7bc6bc10a0019f0fa567/
http://autobuild.buildroot.net/results/4f9/4f904a2913984201ad0bffb63664019c73e0eb77/
http://autobuild.buildroot.net/results/65c/65c7d47fdc0f55fb8deb77e34063320e2af337ee/
http://autobuild.buildroot.net/results/aaa/aaaa1fc92b6a885d223681f40c5df7a962ee1fce/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>