Reduce compile warnings

[ksss]

We found compile error on CLANGARM64 https://github.com/ruby/ruby/actions/runs/22716795417/job/65868368087#step:9:1106

../../../../../../src/.bundle/gems/rbs-3.10.3/ext/rbs_extension/../../src/parser.c:600:9: warning: jump from switch statement to this case label is incompatible with C++ [-Wjump-misses-init]
  600 |         default:
      |         ^
../../../../../../src/.bundle/gems/rbs-3.10.3/ext/rbs_extension/../../src/parser.c:580:41: note: jump bypasses variable initialization
  580 |             rbs_types_function_param_t *param = NULL;
      |                                         ^
../../../../../../src/.bundle/gems/rbs-3.10.3/ext/rbs_extension/../../src/parser.c:591:13: warning: jump from switch statement to this case label is incompatible with C++ [-Wjump-misses-init]
  591 |             KEYWORD_CASES
      |             ^
../../../../../../src/.bundle/gems/rbs-3.10.3/ext/rbs_extension/../../src/parser.c:63:5: note: expanded from macro 'KEYWORD_CASES'
   63 |     case k__TODO__:     \
      |     ^
../../../../../../src/.bundle/gems/rbs-3.10.3/ext/rbs_extension/../../src/parser.c:580:41: note: jump bypasses variable initialization
  580 |             rbs_types_function_param_t *param = NULL;
      |                                         ^

This compilation error can be reproduced by intentionally adding -Wjump-misses-init to the compilation options.

Additionally, I found that -Wreturn-type was also occurring during my investigation, so I have fixed it as well.