Fix file paths added when scanning with sysroot
This fixes a regression introduced by 76f88e78. When
using FONTCONFIG_SYSROOT=/home/tom/test_fonts, strace
reveals that fontconfig attempted to open /test_fonts/Ahem.ttf
instead of /home/tom/test_fonts/Ahem.ttf. The fix is to use
s_dir
instead of dir
in file_prefix
.