Add plaintext.proto

Update Makefile to handle the import in `plaintext.proto`.
Import path is modified to be relative to the project root.
And we run `protoc` from where `Makefile` locates, i.e. the project
root.

Reference:
- plaintext.proto: 62b2c6c482/sec/insecure/pb/plaintext.proto

libp2p/security/insecure/pb/plaintext.proto

@@ -0,0 +1,10 @@
+syntax = "proto2";
+
+package plaintext.pb;
+
+import "libp2p/crypto/pb/crypto.proto";
+
+message Exchange {
+    optional bytes id = 1;
+    optional crypto.pb.PublicKey pubkey = 2;
+}