[btls] Add MonoPInvokeCallback attribute on all full aot profiles (#4879)

[btls] Add MonoPInvokeCallback attribute on all full aot profiles (#4879)

It was #ifdef'ed to only be available on monotouch before,
but the testing_aot_full profile in CI runs into this test failure:

```
MESSAGE:
System.ExecutionEngineException : Attempting to JIT compile method '(wrapper native-to-managed) Mono.Btls.MonoBtlsSslCtx:NativeVerifyCallback (intptr,int,intptr)' while running in aot-only mode. See https://developer.xamarin.com/guides/ios/advanced_topics/limitations/ for more information.

+++++++++++++++++++
STACK TRACE:
  at (wrapper managed-to-native) System.Object:__icall_wrapper_mono_thread_interruption_checkpoint ()
  at (wrapper managed-to-native) System.Runtime.InteropServices.Marshal:GetFunctionPointerForDelegateInternal (System.Delegate)
  at System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate[TDelegate] (TDelegate d) [0x00013] in <fa55a8c31ba0490383239e5850d81686>:0
  at Mono.Btls.MonoBtlsSslCtx..ctor (Mono.Btls.MonoBtlsSslCtx+BoringSslCtxHandle handle) [0x00059] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Btls.MonoBtlsSslCtx..ctor () [0x0000b] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Btls.MonoBtlsContext.InitializeConnection () [0x00000] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Btls.MonoBtlsContext.StartHandshake () [0x00000] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake (Mono.Net.Security.AsyncProtocolRequest asyncRequest, Mono.Net.Security.AsyncOperationStatus status) [0x00004] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.AsyncProtocolRequest.ProcessOperation (Mono.Net.Security.AsyncOperationStatus status) [0x0006b] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.AsyncProtocolRequest.ProcessOperation () [0x0000d] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.AsyncProtocolRequest.StartOperation () [0x0003c] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.AsyncProtocolRequest.StartOperation (Mono.Net.Security.AsyncOperation operation) [0x00024] in <18810f39bffc41dcb0263eb2043cc10e>:0
  at Mono.Net.Security.MobileAuthenticatedStream.ProcessAuthentication (System.Net.LazyAsyncResult lazyResult) [0x00057] in <18810f39bffc41dcb0263eb2043cc10e>:0
```

The attribute itself is already conditional on MONOTOUCH and
FULL_AOT_RUNTIME so simply removing the ifdef should fix the issue.