cmake.bbclass: avoid treating imports as system includes

cmake.bbclass: avoid treating imports as system includes

CMake sets all imported headers as system headers. This causes trouble for c++
projects [1].

Thanks to Jack Mitchell for pointing to the setting [2]. Build tested upon
meta-qt5-extra-world which had lots of fallout before.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
[2] http://lists.openembedded.org/pipermail/openembedded-core/2016-September/126067.html

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>