Move include/wx/unix/execute.h to include/wx/unix/private.

This header is private and is not supposed to be used from the outside the
library.

See #16325.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76683 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Avoid g++ warning about using possibly uninitialized PangoContext.

Initialize it to NULL as it could indeed be left uninitialized in some build
configurations before.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76684 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775