This commit fixes compiler warnings in whisper.cpp by changing the type
of the loop index variable from int64_t to size_t.

Currently the following warnings are generated by the compiler:
```console
/whisper.cpp/src/whisper.cpp:209:27: warning: comparison of integers of different signs: 'int64_t' (aka 'long long') and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  209 |     for (int64_t i = 0; i < nels; ++i) {
      |                         ~ ^ ~~~~
/whisper.cpp/src/whisper.cpp:219:27: warning: comparison of integers of different signs: 'int64_t' (aka 'long long') and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  219 |     for (int64_t i = 0; i < nels; ++i) {
      |                         ~ ^ ~~~~
```