gpu: correct extern declarations

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu/src/command_encoder.zig

@@ -6,7 +6,7 @@ pub const CommandEncoder = enum(usize) {
     // TODO: verify there is a use case for nullable value of this type.
     pub const none: CommandEncoder = @intToEnum(CommandEncoder, 0);
 
-    pub const Descriptor = struct {
+    pub const Descriptor = extern struct {
         next_in_chain: *const ChainedStruct,
         label: ?[*:0]const u8 = null,
     };