(trivial) dixfontstr.h: fix driver FTBS due silent dependency on libxfont2
This header includes libxfont2.h, but the dependency isn't stated anywhere, causing some drivers to FTBS (when libxont2.h is in non-standard location).
Since this header doesn't seem to need including libxfont2.h at all, just stop including it, instead of adding yet another dependency to server SDK.
Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net