uhttpd: fix TCP_FASTOPEN related compile error

Fixes [1]:

  .../uhttpd-a8bf9c00842224edb394e79909053f7628ee6a82/listen.c: In function 'uh_setup_listeners':
  .../uhttpd-a8bf9c00842224edb394e79909053f7628ee6a82/listen.c:120:30: error: 'TCP_FASTOPEN' undeclared (first use in this function)

[1] http://autobuild.buildroot.net/results/56e/56e0727ccd1255b05e03d1b79dc238bd88701230



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>