rtc-tools: rtc-sync needs threads support

rtc-tools: rtc-sync needs threads support

Fixes:
http://autobuild.buildroot.net/results/573/57350271eff9284a8b07ceef02a9960f3568a0a3/
http://autobuild.buildroot.net/results/b6c/b6cf05deab77c7a84c721c95d9d618b1ddc2957e/
http://autobuild.buildroot.net/results/187/1877cfbbe37ef15c16cec5d6ad6e3d4d60bc3cbc/

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>