Only include <unistd.h> under Unix in wxGTK code.

Also remove the inclusions of some Unix-specific headers that were not needed
at all.

Closes #14048.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/utilsgtk.cpp

@@ -45,8 +45,9 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
-#include <sys/wait.h>   // for WNOHANG
+#ifdef __UNIX__
 #include <unistd.h>
+#endif
 
 #if wxUSE_DETECT_SM
     #include <X11/SM/SMlib.h>