Fix build with -Werror

The actual warning is:
src/gui/text/qharfbuzzng.cpp:589:67: error: cast from ‘void*’ \
to ‘uint {aka unsigned int}’ loses precision [-fpermissive]

Change-Id: I75cc7bad2147cd5e5e7c17bc22fe613280e86755
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>