go-cloud-debug-agent: add go1.7 build tag
go-cloud-debug-agent: add go1.7 build tag

debuglet.go won't compile under go 1.6 because of changes in
golang.org/x/debug/server.

Change-Id: Iba798396740bbe9817403c397045ce9b39fc91fe
Reviewed-on: https://code-review.googlesource.com/16690
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tuo Shan <e38988508c7dd9b59ee1f5735040284711960df8@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go