Fix build break after r292225 for Playstation.

[Jigen Zhou]

The following errors occur for Playstation build after r292225:

C:/work/webkit/jenkins/neko/Source/WebKit/Platform/IPC/unix/IPCSemaphoreUnix.cpp:143:12: error: member access into incomplete type 'IPC::Encoder'
    encoder.addAttachment(Attachment(duplicate));
           ^
C:/work/webkit/jenkins/neko/Source/WebKit/Platform/IPC\IPCSemaphore.h:42:7: note: forward declaration of 'IPC::Encoder'
class Encoder;
      ^
C:/work/webkit/jenkins/neko/Source/WebKit/Platform/IPC/unix/IPCSemaphoreUnix.cpp:148:30: error: member access into incomplete type 'IPC::Decoder'
    auto attachment = decoder.takeLastAttachment();
                             ^
C:/work/webkit/jenkins/neko/Source/WebKit/Platform/IPC\IPCSemaphore.h:41:7: note: forward declaration of 'IPC::Decoder'
class Decoder;
      ^
2 errors generated.

[Jigen Zhou]

Created attachment 456399 [details]
Patch

[Jigen Zhou]

Created attachment 456403 [details]
Patch

[EWS]

Committed r292248 (249146@main): <https://commits.webkit.org/249146@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 456403 [details].

[Radar WebKit Bug Importer]

<rdar://problem/91190868>