Fix build build break with bazel 0.25 (#316)

We were picking up ABSL flags transitively.
After bazelbuild/bazel#5636 this no longer works.
load() the flags directly from ABSL's GENERATED_copts.bzl.

Author: jinmel

opencensus/copts.bzl

@@ -22,7 +22,7 @@ We use the same flags as absl, plus turn some warnings into errors.
 """
 
 load(
-    "@com_google_absl//absl:copts/configure_copts.bzl",
+    "@com_google_absl//absl:copts/GENERATED_copts.bzl",
     "ABSL_GCC_FLAGS",
     "ABSL_GCC_TEST_FLAGS",
     "ABSL_LLVM_FLAGS",