kbuild: move the Module.symvers check for external module build

$(objtree)/Module.symvers is not required for descending into
sub-directories. It is needed for the modpost stage.

Move the Module.symvers check to the right place.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1620,10 +1620,10 @@ $(objtree)/Module.symvers:
 
 module-dirs := $(addprefix _module_,$(KBUILD_EXTMOD))
 PHONY += $(module-dirs) modules
-$(module-dirs): prepare $(objtree)/Module.symvers
+$(module-dirs): prepare
 	$(Q)$(MAKE) $(build)=$(patsubst _module_%,%,$@) need-modorder=1
 
-modules: $(module-dirs)
+modules: $(module-dirs) $(objtree)/Module.symvers
 	@$(kecho) '  Building modules, stage 2.';
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost