Fix PlatformIdentifier not having a compiler guard

src/Tgstation.Server.Host/Core/Application.cs

@@ -295,7 +295,6 @@ namespace Tgstation.Server.Host.Core
 			services.AddSingleton<IPasswordHasher<Models.User>, PasswordHasher<Models.User>>();
 
 			// configure platform specific services
-#pragma warning disable CA1416 // Validate platform compatibility
 			if (postSetupServices.PlatformIdentifier.IsWindows)
 			{
 				AddWatchdog<WindowsWatchdogFactory>(services, postSetupServices);
@@ -325,7 +324,6 @@ namespace Tgstation.Server.Host.Core
 
 				services.AddSingleton<IHostedService, PosixSignalHandler>();
 			}
-#pragma warning restore CA1416 // Validate platform compatibility
 
 			// configure component/misc services
 			services.AddScoped<IPortAllocator, PortAllocator>();

src/Tgstation.Server.Host/System/IPlatformIdentifier.cs

@@ -1,4 +1,6 @@
-namespace Tgstation.Server.Host.System
+using System.Runtime.Versioning;
+
+namespace Tgstation.Server.Host.System
 {
 	/// <summary>
 	/// For identifying the current platform.
@@ -8,6 +10,7 @@
 		/// <summary>
 		/// If the current platform is a Windows platform.
 		/// </summary>
+		[SupportedOSPlatformGuard("windows")]
 		bool IsWindows { get; }
 
 		/// <summary>

src/Tgstation.Server.Host/System/PlatformIdentifier.cs

@@ -1,4 +1,5 @@
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 
 namespace Tgstation.Server.Host.System
 {
@@ -6,6 +7,7 @@ namespace Tgstation.Server.Host.System
 	sealed class PlatformIdentifier : IPlatformIdentifier
 	{
 		/// <inheritdoc />
+		[SupportedOSPlatformGuard("windows")]
 		public bool IsWindows { get; }
 
 		/// <inheritdoc />