GET WARNING: modpost: missing MODULE_LICENSE() when LKM is Compiled with Multiple Src files

I have a LKM named RtmNetlinkLKM.c and compiles and run fine. The moment I update its Makefile to compile with other src files, it starts giving warning:

WARNING: modpost: missing MODULE_LICENSE()

The following MODULE_LICENSE("GPL"); is already present in kernel module file.

Previous Makefile, module compiles fine

obj-m += RtmNetlinkLKM.oall:    make -C /lib/modules/`uname -r`/build M=$(PWD) modulesclean:    make -C /lib/modules/`uname -r`/build M=$(PWD) clean

updated Makefile, module now compiles with a warning

obj-m += RtmNetlinkLKM.oRtmNetlinkLKM-objs += rt_kern.o gluethread/glthread.o << Added two more sourcesall:    make -C /lib/modules/`uname -r`/build M=$(PWD) modulesclean:    make -C /lib/modules/`uname -r`/build M=$(PWD) clean    rm -f rt_kern.o    rm -f gluethread/glthread.o

When compiled using second Makefile, it gives stated warning. When I add MODULE_LICENSE("GPL") in gluethread/glthread.c , warning goes away. I don't understand, why do I need to add "GPL" license in glthread.c when it is not a module but contain functions to be used by module (It is a linked list mini-library). Why doesn't it complain with other src file rt_kern.c in a similar way. I had never made any changes in original module file RtmNetlink.c throughout this process.