kbuild: external module build warnings when KBUILD_OUTPUT set and W=1

Commit db547ef1 ("Kbuild: don't add obj tree in additional includes")
causes warnings (-Wmissing-include-dirs) when compiling external modules
with KBUILD_OUTPUT set and W=1.  This is because $src can be an absolute
path to the external module source which when prefixed with -I$(srctree)/
generates an incorrect directory path.
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.lib

@@ -155,7 +155,7 @@ else
 # $(call addtree,-I$(obj)) locates .h files in srctree, from generated .c files
 #   and locates generated .h files
 # FIXME: Replace both with specific CFLAGS* statements in the makefiles
-__c_flags	= $(if $(obj),-I$(srctree)/$(src) -I$(obj)) \
+__c_flags	= $(if $(obj),$(call addtree,-I$(src)) -I$(obj)) \
 		  $(call flags,_c_flags)
 __a_flags	= $(call flags,_a_flags)
 __cpp_flags     = $(call flags,_cpp_flags)