Fix warning in generate.cc

[flaviens]

Hi there!

This commit proposes to fix the warning:

scripts/generate.cc: In function ‘void print_expression(FILE*, int, uint32_t, bool, bool, bool)’:
scripts/generate.cc:157:31: warning: left shift of negative value [-Wshift-negative-value]
  157 |         while ((mask & ~((~0) << num_modes)) == 0)
      |                          ~~~~~^~~~~~~~~~~~

that I got when using g++.

Thank you!
Flavien