Fix vector operations benchmark compilation

Some of the benchmark functions use the same name as other functions
in the ceres namespace. For example Axpby defines both benchmark but
also an utility function in eigen_vector_ops.h. It seems to confuse
some compilers and leads to a compilation error rooting deeper into
the benchmark header itself: it seems that the compiler can not
deduct which of the instances of such functions to use.

Wrapping the file into an anonymous namespace solves the problem.
Alternative could be to use benchmark namespace to make thins more
explicit, for example ceres::internal::benchmark.

Tested on the following configuration:
- macOS 15.4
- Xcode 16.3
- Apple M3 CPU
- google-benchmark 1.9.2 installed via homebrew

Change-Id: Id127015dd22de99c6c3da88e71f255736e0bed82

internal/ceres/parallel_vector_operations_benchmark.cc

diff --git a/internal/ceres/parallel_vector_operations_benchmark.cc b/internal/ceres/parallel_vector_operations_benchmark.cc
index b658ea5..c3dcdb4 100644
--- a/internal/ceres/parallel_vector_operations_benchmark.cc
+++ b/internal/ceres/parallel_vector_operations_benchmark.cc
@@ -34,6 +34,10 @@
 #include "ceres/parallel_for.h"
 
 namespace ceres::internal {
+// Wrap everything into an anonymous namespace to guide deduction of possibly
+// polymorphic functions that define benchmark and which could be defined in the
+// ceres namespace.
+namespace {
 // Older versions of benchmark library (for example, one shipped with
 // ubuntu 20.04) do not support range generation and range products
 #define VECTOR_SIZES(num_threads)    \
@@ -321,7 +325,7 @@
   CHECK_GT(z.squaredNorm(), 0.);
 }
 BENCHMARK(AxpbyParallel)->VECTOR_SIZE_THREADS;
-
+}  // namespace
 }  // namespace ceres::internal
 
 BENCHMARK_MAIN();