ex_voxelBlockManager_host_cuda: add missing <string> include

MSVC does not pull in std::stoi/stof transitively; include <string> explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Efty Sifakis <esifakis@nvidia.com>

Author: sifakis

nanovdb/nanovdb/examples/ex_voxelBlockManager_host_cuda/vbm_host_cuda.cpp

@@ -19,6 +19,7 @@
 #include <nanovdb/NanoVDB.h>
 
 #include <random>
+#include <string>
 #include <vector>
 #include <iostream>
 #include <stdexcept>