Increase tolerance in small_blas test for Cygwin.

- Previous tolerance of 2.0 * std::numeric_limits<double>::epsilon()
  was too tight for Cygwin, worked on all other known platforms.

Change-Id: Ia79ad8961272dbb608d8e8ddd3f6d52e5f0735f4

internal/ceres/small_blas_test.cc

diff --git a/internal/ceres/small_blas_test.cc b/internal/ceres/small_blas_test.cc
index 675aec9..ddb11ab 100644
--- a/internal/ceres/small_blas_test.cc
+++ b/internal/ceres/small_blas_test.cc
@@ -37,7 +37,7 @@
 namespace ceres {
 namespace internal {
 
-const double kTolerance = 2.0 * std::numeric_limits<double>::epsilon();
+const double kTolerance = 3.0 * std::numeric_limits<double>::epsilon();
 
 TEST(BLAS, MatrixMatrixMultiply) {
   const int kRowA = 3;