Makefile: enable -Wsparse-error for DEVELOPER build

With -Wsparse-error, "make sparse" would fail, instead of just
giving a warning message.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.dev

@@ -1,5 +1,6 @@
 ifeq ($(filter no-error,$(DEVOPTS)),)
 DEVELOPER_CFLAGS += -Werror
+SPARSE_FLAGS += -Wsparse-error
 endif
 ifneq ($(filter pedantic,$(DEVOPTS)),)
 DEVELOPER_CFLAGS += -pedantic