linknx: select libcurl to fix autoreconf

linknx: select libcurl to fix autoreconf

linknx configure.ac uses the LIBCURL_CHECK_CONFIG macro that is only defined
in libcurl.m4 that is part of libcurl. Select libcurl for now to fix
autoreconf.

Fixes:
http://autobuild.buildroot.net/results/96e/96e9ae0df7ebb7ad3d748a6b518cc59570e0b995/
http://autobuild.buildroot.net/results/179/17971c9f8925ff8623a7d97aac8eda0c287a76c9/
http://autobuild.buildroot.net/results/2a8/2a8af93d6439b313b048ccac7b6cfecbb6baff55/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>