Define wxUSE_CAIRO in wx/chkconf.h to ensure that it's always defined.

Define wxUSE_CAIRO in wx/chkconf.h to ensure that it's always defined.

wxUSE_CAIRO used to be defined only in wx/cairo.h but this header wasn't
included by src/common/dcgraph.cpp so the code there was compiled as if we
were not using Cairo even when we were (thanks g++ for the warning).

Define it in wx/chkconf.h (included from wx/defs.h, i.e. always) now to ensure
that not only this bug is fixed but also that it can't happen any more.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775