Install private headers used by wx/unix/execute.h.

Although private headers are not normally installed, do install the ones
included from the (erroneously) public wx/unix/execute.h to allow code
including it to compile.

Closes #16325.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76685 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775