kbuild: Remove all cscope files during mrproper

From: GOTO Masanori <gotom@debian.or.jp>

cscope generaltes various files, with different extension.
Make sure to delete all cscope files during mrproper

Makefile

@@ -753,7 +753,7 @@ MRPROPER_FILES += \
 	.menuconfig.log \
 	include/asm \
 	.hdepend include/linux/modversions.h \
-	tags TAGS cscope.out kernel.spec \
+	tags TAGS cscope* kernel.spec \
 	.tmp*
 
 # Directories removed with 'make mrproper'