cflags: disable -Werror=date-time

This has been causing issues with developers using older
compilers. Until we are on the same version and have a
better way to deal with this, lets disable it.

Change-Id: I25c895c72a7a1a888332320d78bd47246dbafaff
Signed-off-by: Anas Nashif <anas.nashif@intel.com>