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.

(From OE-Core rev: 4e0daf05ed04e9fb4343032c6290a379f53b89f3)

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>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-kernel/linux/linux-yocto/noslang.patch

@@ -1,20 +0,0 @@
-We (OE) install slang into /usr/include so we never need to look into 
-/usr/include/slang/. We never want to look into a hardcoded path like this
-since it triggers host infection issues. For now, simply remove this
-since it causes us problems.
-
-Upstream-Status: Pending (would need rework)
-
-Index: tools/perf/Makefile
-===================================================================
---- linux.orig/tools/perf/Makefile	2012-08-07 10:29:43.020149620 +0000
-+++ linux/tools/perf/Makefile	2012-08-07 10:30:08.128148098 +0000
-@@ -504,7 +504,7 @@
- 		BASIC_CFLAGS += -DNO_NEWT_SUPPORT
- 	else
- 		# Fedora has /usr/include/slang/slang.h, but ubuntu /usr/include/slang.h
--		BASIC_CFLAGS += -I/usr/include/slang
-+		# BASIC_CFLAGS += -I/usr/include/slang
- 		EXTLIBS += -lnewt -lslang
- 		LIB_OBJS += $(OUTPUT)util/ui/setup.o
- 		LIB_OBJS += $(OUTPUT)util/ui/browser.o