breakpad.bbclass: set includedir in CXXFLAGS

Previously this was only set in CFLAGS.
An build issue was discovered while convering in internal application
from Qt4 to Qt5. As a result of this, the required header files were
no longer found, until we also set CXXFLAGS.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>