Fix size of HB_FontRec in qharfbuzz_copy_p.h

HB_FontRec was defined in qharfbuzz_copy_p.h as a void* typedef,
which is incorrect, and is the wrong size. This caused things
accessing QFontEngine data after an HB_FontRec to read from the
wrong offset. The visible consequence of this was incorrect text
rendering in OpenGL in Linux (QTBUG-29966).