tools: cmake: link librt if needed (FS#381)

Patch cmakes cmcurl component to unconditionally link librt when we discover
a clock_gettime() symbol in -lrt.

This is required to sucessfully link LibreSSL's libcrypto which might or
might not reqauire librt.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>