Pick dependency files in submodules. (sonic-net#15142)

#### Why I did it

Failed to build sonic-dhcp6relay_1.0.0-0_amd64.deb

#### How I did it

src/dhcprelay has git submodule.
Dependency files by "git ls-files" are not picked files in submodules.
Add --recurse-submodules, work again.

#### How to verify it

make all

rules/dhcprelay.dep

@@ -2,7 +2,7 @@
 SPATH       := $($(SONIC_DHCPRELAY)_SRC_PATH)
 DEP_FILES   := $(SONIC_COMMON_FILES_LIST) rules/dhcprelay.mk rules/dhcprelay.dep   
 DEP_FILES   += $(SONIC_COMMON_BASE_FILES_LIST)
-SMDEP_FILES := $(addprefix $(SPATH)/,$(shell cd $(SPATH) && git ls-files))
+SMDEP_FILES := $(addprefix $(SPATH)/,$(shell cd $(SPATH) && git ls-files --recurse-submodules))
 
 $(SONIC_DHCPRELAY)_CACHE_MODE  := GIT_CONTENT_SHA 
 $(SONIC_DHCPRELAY)_DEP_FLAGS   := $(SONIC_COMMON_FLAGS_LIST)