synergy: needs gcc >= 4.9

synergy: needs gcc >= 4.9

Since version 1.9.0 and
https://github.com/symless/synergy-core/commit/c0376e9e2f659aec23a748f9a8d8ee6cd82f1281,
synergy needs C++14 so add a dependency on
BR2_TOOLCHAIN_GCC_AT_LEAST_4_9

Fixes:
 - http://autobuild.buildroot.org/results/c4646ee9342ea8bd906bfe2b29996c48cb403ccc

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Reviewed-by: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>