Fix unused parameter compiler warnings in numeric_diff.h

Change-Id: I2b35babba17229387c6d346c46a2c6960db96e47

include/ceres/internal/numeric_diff.h

diff --git a/include/ceres/internal/numeric_diff.h b/include/ceres/internal/numeric_diff.h
index da04a8d..b9aab88 100644
--- a/include/ceres/internal/numeric_diff.h
+++ b/include/ceres/internal/numeric_diff.h
@@ -204,6 +204,15 @@
       const int parameter_block_size,
       double **parameters,
       double *jacobian) {
+    // Silence unused parameter compiler warnings.
+    (void)functor;
+    (void)residuals_at_eval_point;
+    (void)relative_step_size;
+    (void)num_residuals;
+    (void)parameter_block_index;
+    (void)parameter_block_size;
+    (void)parameters;
+    (void)jacobian;
     LOG(FATAL) << "Control should never reach here.";
     return true;
   }