Fix #1924: Emit SPV_EXT_physical_storage_buffer, not the KHR form.

The KHR form comes along as part of 1.5, and otherwise should require
another form of request to emit it.
diff --git a/SPIRV/GlslangToSpv.cpp b/SPIRV/GlslangToSpv.cpp
index afb405e..8760f4a 100644
--- a/SPIRV/GlslangToSpv.cpp
+++ b/SPIRV/GlslangToSpv.cpp
@@ -1399,7 +1399,7 @@
 
     if (glslangIntermediate->usingPhysicalStorageBuffer()) {
         addressingModel = spv::AddressingModelPhysicalStorageBuffer64EXT;
-        builder.addIncorporatedExtension(spv::E_SPV_KHR_physical_storage_buffer, spv::Spv_1_5);
+        builder.addIncorporatedExtension(spv::E_SPV_EXT_physical_storage_buffer, spv::Spv_1_5);
         builder.addCapability(spv::CapabilityPhysicalStorageBufferAddressesEXT);
     };
     if (glslangIntermediate->usingVulkanMemoryModel()) {