src/poky:sanity: Require gcc 6 or later

New versions of rpm have issues with the libgomp support in gcc 5.x
so raise the minimum to 6 or later. This mainly affects Ubuntu 16.04.

(From OE-Core rev: aca2e5816203b54e0955eaa99fc980d010052d5d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>