glfw: correctly @errorSetCast when returning errors from createWindowSurface

Fixes an issue where `createWindowSurface` would not compile (found when
updating https://github.com/hexops/mach-glfw-vulkan-example to latest.)

Introduced in hexops/mach#115

cc @InKryption

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

glfw/src/vulkan.zig

@@ -229,7 +229,7 @@ pub inline fn createWindowSurface(vk_instance: anytype, window: Window, vk_alloc
         Error.InvalidValue => @panic("Attempted to use window with client api to create vulkan surface."),
         Error.APIUnavailable,
         Error.PlatformError,
-        => err,
+        => @errSetCast(error{ APIUnavailable, PlatformError }, err),
         else => unreachable,
     };
     return v;