Different signedness comparison warning

[dimalvovs]

Match the type of integers to avoid potential problems flagged in the warning below that gets generated during build:

src/bindings.cpp: In lambda function:
src/bindings.cpp:309:35: warning: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Wsign-compare]
  309 |                 for (int i = 0; i < m.nRow(); i++) {
      |                                 ~~^~~~~~~~~~
src/bindings.cpp:310:39: warning: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Wsign-compare]
  310 |                     for (int j = 0; j < m.nCol(); j++) {
      |                                     ~~^~~~~~~~~~