Fix build with more recent libmodplug.

Modern versions of libmodplug no longer put -I${includedir}/libmodplug
in the pkg-config file, which breaks our files that #include <modplug.h>.

Seems the expectation is for packges to #include <libmodplug/modplug.h>
instead, which should work totally fine with old versions too.  The new
libmodplug helper macro in dxcommon uses that, so just update the the
rest of the code accordingly.