fp-print: Fix sign-compare warnings

libfprint/fp-print.c: In function ‘fp_print_equal’:
libfprint/fp-print.c:596:21: warning: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘guint’ {aka ‘unsigned int’} [-Wsign-compare]
  596 |       for (i = 0; i < self->prints->len; i++)
      |                     ^
libfprint/fp-print.c: In function ‘fp_print_serialize’:
libfprint/fp-print.c:667:21: warning: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘guint’ {aka ‘unsigned int’} [-Wsign-compare]
  667 |       for (i = 0; i < print->prints->len; i++)
      |                     ^
libfprint/fp-print.c: In function ‘fp_print_deserialize’:
libfprint/fp-print.c:823:21: warning: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘gsize’ {aka ‘long unsigned int’} [-Wsign-compare]
  823 |       for (i = 0; i < g_variant_n_children (prints); i++)
      |                     ^