kernel.bbclass: fix INC directory for SLANG

kernel.bbclass: fix INC directory for SLANG

In the kernel's tools/perf/Makefile CFLAGS was previously hard coded to
contain "-I/usr/include/slang" to work with hosts that have
"/usr/include/slang/slang.h" as well as hosts that have
"/usr/include/slang.h". This path can cause compile warnings like:

  cc1: warning: '/usr/include/slang' doesn't exists.

  or

  cc1: warning: include location "/usr/include/slang" is unsafe for
  cross-compilation [-Wpoison-system-directories]

Then in some cases warnings become errors if WERROR is enabled hence
build errors.

In coordination with a kernel fix, we can fix this error for all
kernels by modifying the perf Makefile within the staged kernel
source.

Signed-off-by: Liang Li <liang.li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>