Fixed sign comparison warning

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/10280498668/job/28447977337#step:11:240

src/_imagingft.c:1246:23: warning: comparison of integers of different signs: 'int' and 'FT_UInt' (aka 'unsigned int') [-Wsign-compare]
for (int i = 0; i < num_namedstyles; i++) {
~ ^ ~~~~~~~~~~~~~~~
1 warning generated.
src/_imagingft.c:1246:23: warning: comparison of integers of different signs: 'int' and 'FT_UInt' (aka 'unsigned int') [-Wsign-compare]
for (int i = 0; i < num_namedstyles; i++) {
~ ^ ~~~~~~~~~~~~~~~