Unbreak the build with EIGENSPARSE is disabled

Change-Id: Ia3a6121f031e647b51adba427c814e818eed2d2d

internal/ceres/eigensparse.h

diff --git a/internal/ceres/eigensparse.h b/internal/ceres/eigensparse.h
index 1ab1c57..58d15eb 100644
--- a/internal/ceres/eigensparse.h
+++ b/internal/ceres/eigensparse.h
@@ -95,6 +95,17 @@
 
 }  // namespace ceres::internal
 
+#else
+
+namespace ceres::internal {
+
+class EigenSparse {
+ public:
+  static constexpr bool IsNestedDissectionAvailable() noexcept { return false; }
+};
+
+}  // namespace ceres::internal
+
 #endif  // CERES_USE_EIGEN_SPARSE
 
 #endif  // CERES_INTERNAL_EIGENSPARSE_H_