shared/bootspec: avoid signed-unsigned comparison

../src/shared/bootspec.c: In function ‘find_sections’:
../src/shared/bootspec.c:425:23: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘int’} and ‘uint32_t’ {aka ‘unsigned int’} [-Wsign-compare]
  425 |                 if (n != size)
      |                       ^~