kbuild: Remove cscope.out during make mrproper

From: "Nathan T. Lynch" <ntl@pobox.com>

The attached patch fixes the toplevel Makefile to remove cscope.out
during make mrproper.  The default name for the database that cscope
creates is cscope.out, and this is what the cscope rule in the
makefile uses.  Currently, mrproper will leave cscope.out behind,
which can make for interesting diffs...

Makefile

@@ -675,7 +675,7 @@ MRPROPER_FILES += \
 	.menuconfig.log \
 	include/asm \
 	.hdepend include/linux/modversions.h \
-	tags TAGS cscope kernel.spec \
+	tags TAGS cscope.out kernel.spec \
 	.tmp*
 
 # Directories removed with 'make mrproper'