Fix: Compilation Error When Compiling with Vulkan

This PR fixes an compilation error where sstream used to be included
on a higher level, but has been removed and we need to include them
where they are actually used.

This wasn't detected as most developers don't have the vulkan backend
active.

Pull Request: https://projects.blender.org/blender/blender/pulls/111203

source/blender/gpu/vulkan/vk_debug.cc

@@ -6,6 +6,8 @@
  * \ingroup gpu
  */
 
+#include <sstream>
+
 #include "BKE_global.h"
 #include "CLG_log.h"
 

source/blender/gpu/vulkan/vk_shader.cc

@@ -6,6 +6,8 @@
  * \ingroup gpu
  */
 
+#include <sstream>
+
 #include "vk_shader.hh"
 
 #include "vk_backend.hh"