package/openswan: fix parallel build

Fixes:
    http://autobuild.buildroot.org/results/2ec/2ecb098b01bd44d75182b15f98c1fd2a8c927438/
    http://autobuild.buildroot.org/results/18f/18f0166f3fa831f321818e286d9c2e9037f8c8d0/


    ...

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>