swupdate: switch to local config librt dependency

swupdate: switch to local config librt dependency

Upstream rejected[1] the patch added in commit 6a1aa4e2daf0 (swupdate: fix
build with older glibc). Instead of carrying this patch going forward, make
librt an extra library dependency in the Buildroot local swupdate .config.

This solution is somewhat less optimal than the patch, because it always adds
librt regardless of whether SURICATTA_HAWKBIT is enabled. In theory we could
add a test for (BR2_PACKAGE_JSON_C && BR2_PACKAGE_LIBCURL), which are the
SURICATTA_HAWKBIT dependencies, and edit .config at build time. But this adds
much complexity for little gain.

[1] https://groups.google.com/forum/?hl=en#!topic/swupdate/77QWIka27es

Cc: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>