make: lib -- use ldflags-so and obj-ext-src-y

We need ldflags-so to link libcriu with -lprotobuf-c
and rpc.pb-c.o to use RPC.
Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

lib/Makefile

 lib-so			+= libcriu
 obj-y			+= criu.o
+obj-ext-src-y		+= protobuf/rpc.pb-c.o
 
-includes		+= -iquote $(obj)/../arch/$(ARCH)/include -iquote $(obj)/../include -iquote $(obj)/..
+includes		+= -iquote $(obj)/../arch/$(ARCH)/include -iquote $(obj)/../include -iquote $(obj)/.. -iquote $(obj)/../protobuf
 cflags-y		+= $(includes) -fPIC -Wa,--noexecstack -fno-stack-protector
 cflags-so		+= -rdynamic
+ldflags-so		+= -lprotobuf-c
 
 .SECONDARY: