cmake: drop -fpermissive

cmake: drop -fpermissive

* it was dropped from default CXXFLAGS in:
  commit 24dd8e129447013ee98609f3892ec414b1b21340
  Author: Richard Purdie <richard.purdie@linuxfoundation.org>
  Date:   Sun Mar 2 17:38:33 2014 +0000

    bitbake.conf: Drop -fpermissive

    Drop the -fpermissive C++ compiler flag. We've had this around for years, most
    code should have been fixed long ago. Its possible some recipes may fail
    however we can (and should) just use the flag where needed.

* I haven't build world with this yet, but maybe it's time to
  drop it here as well at least for consistency

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>