Fix signature error on FT_Glyph_Get_CBox causing memory access error

Hi,

I've tried to use Glyph.GetCBox() and I had a memory access error.
While looking at SharpFont code I saw that there was an error on FT_Glyph_Get_CBox definition (see http://www.freetype.org/freetype2/docs/reference/ft2-glyph_management.html#FT_Glyph_Get_CBox).
I've fixed it and now it works !