gpu: correct extern declarations

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

gpu/src/command_buffer.zig

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

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,
     };

gpu/src/dawn.zig

@@ -1,13 +1,13 @@
 const ChainedStruct = @import("types.zig").ChainedStruct;
 const Texture = @import("texture.zig").Texture;
 
-pub const CacheDeviceDescriptor = struct {
+pub const CacheDeviceDescriptor = extern struct {
     // TODO: file an issue on Dawn: why not named nextInChain?
     chain: ChainedStruct,
     isolation_key: [*:0]const u8,
 };
 
-pub const EncoderInternalUsageDescriptor = struct {
+pub const EncoderInternalUsageDescriptor = extern struct {
     // TODO: file an issue on Dawn: why not named nextInChain?
     chain: ChainedStruct,
     use_internal_usages: bool,

gpu/src/main.zig

@@ -46,7 +46,7 @@ pub const RenderPassTimestampLocation = @import("types.zig").RenderPassTimestamp
 pub const PowerPreference = @import("types.zig").PowerPreference;
 pub const RenderPassColorAttachment = @import("types.zig").RenderPassColorAttachment;
 
-pub const ComputePassTimestampWrite = struct {
+pub const ComputePassTimestampWrite = extern struct {
     query_set: QuerySet,
     query_index: u32,
     location: ComputePassTimestampLocation,