Adds missing source and tests to bazel build.

Change 12060 added parallel_for_tbb.cc and parallel_for_test.cc but did not add
it to the bazel build rule.

Tested by running bazel tests.

Change-Id: I286cf0e191e0353f2c0266911a79e01f09e47343

BUILD

diff --git a/BUILD b/BUILD
index 33c8e8d..e488f29 100644
--- a/BUILD
+++ b/BUILD
@@ -125,6 +125,7 @@
     "normal_prior",
     "numeric_diff_cost_function",
     "ordered_groups",
+    "parallel_for",
     "parameter_block_ordering",
     "parameter_block",
     "partitioned_matrix_view",

bazel/ceres.bzl

diff --git a/bazel/ceres.bzl b/bazel/ceres.bzl
index e61650e..e6aac07 100644
--- a/bazel/ceres.bzl
+++ b/bazel/ceres.bzl
@@ -87,6 +87,7 @@
     "low_rank_inverse_hessian.cc",
     "minimizer.cc",
     "normal_prior.cc",
+    "parallel_for_tbb.cc",
     "parameter_block_ordering.cc",
     "partitioned_matrix_view.cc",
     "polynomial.cc",