remove bogus -I/usr/include/slang from CPPFLAGS
    
    Hardcoding -I/usr/include/slang in CPPFLAGS is bogus for
    cross-compilation. With recent versions of slang, the headers are
    installed in ${sysroot}/usr/include directly, so there is no need for
    an additional flag. And if one was needed, it should be added by the
    configure script, after detecting the right header location.
    
    Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
    Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>