native_posix: override C standard version to 2011

Override the C standard used for compilation to C 2011
This is due to some tests using _Static_assert which is a 2011
feature, but otherwise relying on compilers supporting it also
when set to C99.
This was in general ok, but with some host compilers and C
library versions it led to problems. So we override it to 2011
for native_posix board.

Fixes: #8529

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>