package/asterisk: fix parallel build/install

Fixes:

    http://autobuild.buildroot.org/results/7c9892b9425aff6e54b8c8942dbdcb0b26e58572



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