kbuild: Update example for ccflags-y usage

Update the ccflags-y example to match current usage.
Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kbuild/makefiles.txt

@@ -297,8 +297,8 @@ more details, with real examples.
 	ccflags-y specifies options for compiling with $(CC).
 
 	Example:
-		# drivers/acpi/Makefile
-		ccflags-y := -Os
+		# drivers/acpi/acpica/Makefile
+		ccflags-y			:= -Os -D_LINUX -DBUILDING_ACPICA
 		ccflags-$(CONFIG_ACPI_DEBUG)	+= -DACPI_DEBUG_OUTPUT
 
 	This variable is necessary because the top Makefile owns the