Add missing import comment (uber-go#574)

We let one exposed package sneak through without an import comment.
There's no practical way for anyone to be using with the wrong import
path, but let's clean up anyways.

buffer/buffer.go

@@ -21,7 +21,7 @@
 // Package buffer provides a thin wrapper around a byte slice. Unlike the
 // standard library's bytes.Buffer, it supports a portion of the strconv
 // package's zero-allocation formatters.
-package buffer
+package buffer // import "go.uber.org/zap/buffer"
 
 import "strconv"