Work around wrong vsscanf() declaration under HP-UX.

Under this system vsscanf() is declared as taking a non-const char* as first
argument which prevented our code using it from compiling. Wrap it in
wxCRT_VsscanfA() adding the necessary const_cast<> to fix this.

Closes #15638.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75339 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775