Makefile: add QUIET_GEN to "cscope" target

Don't show the very verbose $(FIND_SOURCE_FILES) command on every
"make cscope" invocation.

See my recent 3c80fcb591 (Makefile: add QUIET_GEN to "tags" and "TAGS"
targets, 2021-03-28) for the same fix for the other adjacent targets.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2739,7 +2739,7 @@ tags: FORCE
 
 .PHONY: cscope
 cscope:
-	$(RM) cscope*
+	$(QUIET_GEN)$(RM) cscope* && \
 	$(FIND_SOURCE_FILES) | xargs cscope -b
 
 ### Detect prefix changes