Fix nits. (census-instrumentation#278)

* kVersionLen was unused. Use it.

* Add missing dependency.

Author: g-easy

opencensus/trace/BUILD

@@ -127,6 +127,7 @@ cc_library(
     visibility = ["//visibility:public"],
     deps = [
         ":span_context",
+        "@com_google_absl//absl/strings",
     ],
 )
 

opencensus/trace/internal/grpc_trace_bin.cc

@@ -42,7 +42,8 @@ constexpr int kTraceIdOfs = 1;
 constexpr int kSpanIdOfs = kTraceIdOfs + 1 + kTraceIdLen;
 constexpr int kTraceOptionsOfs = kSpanIdOfs + 1 + kSpanIdLen;
 
-static_assert(1 + 1 + kTraceIdLen + 1 + kSpanIdLen + 1 + kTraceOptionsLen ==
+static_assert(kVersionLen + 1 + kTraceIdLen + 1 + kSpanIdLen + 1 +
+                      kTraceOptionsLen ==
                   kGrpcTraceBinHeaderLen,
               "header length is wrong");