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/Test/baseResults/spv.bufferhandle3.frag.out b/Test/baseResults/spv.bufferhandle3.frag.out
index b888fe4..c02c34c 100644
--- a/Test/baseResults/spv.bufferhandle3.frag.out
+++ b/Test/baseResults/spv.bufferhandle3.frag.out
@@ -5,7 +5,7 @@
 
                               Capability Shader
                               Capability PhysicalStorageBufferAddressesEXT
-                              Extension  "SPV_KHR_physical_storage_buffer"
+                              Extension  "SPV_EXT_physical_storage_buffer"
                               Extension  "SPV_KHR_storage_buffer_storage_class"
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel PhysicalStorageBuffer64EXT GLSL450