tools/cppcheck_wrapper.py: return code fix

The cppcheck wrapper was failing to trigger a
test fail in ci_cmake.py because it was not returning
cppcheck's return code. Now this return code is returned
as the wrapper exit code, solving this.

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: I58e5ec5225f366f986dd3a16d4ef170b97f89796