Update CODEOWNERS for IAST owned file (#6358)

## Summary of changes

Add ASM owner for generated callsite file

## Reason for change

ASM owns this file

Author: andrewlock

.github/CODEOWNERS

@@ -86,6 +86,7 @@
 # ASM
 /tracer/src/Datadog.Trace/AppSec/         @DataDog/asm-dotnet
 /tracer/src/Datadog.Tracer.Native/iast/    @DataDog/asm-dotnet
+/tracer/src/Datadog.Tracer.Native/Generated/generated_callsites.g.h     @DataDog/asm-dotnet
 /tracer/test/test-applications/security/   @DataDog/asm-dotnet
 /tracer/test/Datadog.Trace.Security.IntegrationTests/ @DataDog/asm-dotnet
 /tracer/test/Datadog.Trace.Security.Unit.Tests/       @DataDog/asm-dotnet