Update PdfPage.java

Hey,

First of all, thanks a lot for that tremendous component you provided.

I ran into a regular crash when i was using the component to display various pdf file : on the left pane the name of the file and when the user click on a name, the pdf is displayed on the right.

Switching quickly between files to display led me into a RunTimeException in the function : 

static PdfPage createPage(long dochandle, int pageno)
    {
        return new PdfPage(open(dochandle, pageno), dochandle);
    }

in org.vudroid.pdfdroid.codec.PdfPage

A little investigation led me to find that this happened when two threads was calling this function at the same time.

Just added a "synchronized" qualifier on the function and that bug was gone.