DLL.h: remove distinction between QPDF_DLL_CLASS and QPDF_DLL for mingw
DLL.h: remove distinction between QPDF_DLL_CLASS and QPDF_DLL for mingw

jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf,qpdf/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,jberkenbilt/qpdf,qpdf/qpdf,qpdf/qpdf