package/linknx: add optional dependency to libcurl again

package/linknx: add optional dependency to libcurl again

https://git.buildroot.net/buildroot/commit/package/linknx?id=e1c9a2349006e657e76dff35706a774376921fb7
made libcurl a hard dependency to fix autoreconf.

https://git.buildroot.net/buildroot/commit/package/linknx?id=b56083099f113817edc6811e2cdd820df1c80450
removed autoreconf but did not re-add libcurl as optional dependency by
mistakenly assuming that libcurl is needed only for autoreconf.

Tested using this defconfig:

BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_LIBCURL=y
BR2_PACKAGE_LINKNX=y

Configure log of linknx without this patch

checking for curl-config... /usr/bin/curl-config
checking for the version of libcurl... 7.52.1
checking for libcurl >= version 7.14.0... yes
checking whether libcurl is usable... no

and with this patch

checking for curl-config... /home/buildroot/br4/output/host/i586-buildroot-linux-uclibc/sysroot/usr/bin/curl-config
checking for the version of libcurl... 7.57.0
checking for libcurl >= version 7.14.0... yes
checking whether libcurl is usable... yes
checking for curl_free... yes

Inspired by the fli4l project
https://web.nettworks.org/repo/changelog/fli4l?cs=49347

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