internal/compression/grpc: use import block for blank import (#402)

Just encountered golang/lint#366
whose work around is to convert a single blank import "_"
into a block import.

Author: Bogdan Drutu

internal/compression/grpc/gzip.go

@@ -14,4 +14,7 @@
 
 package grpc
 
-import _ "google.golang.org/grpc/encoding/gzip" // import the gzip package with auto-registers the gzip grpc compressor
+import (
+	// import the gzip package with auto-registers the gzip grpc compressor
+	_ "google.golang.org/grpc/encoding/gzip"
+)