Make value_type.h compatible with -Wconversion

[casperisfine]

Followup: #10843

[Feature #20507]

This was missed from the initial commit.

../../.././include/ruby/internal/value_type.h:446:27: error: implicit conversion changes signedness: 'enum ruby_value_type' to 'int' [-Werror,-Wsign-conversion]
    rb_unexpected_type(v, t);
    ~~~~~~~~~~~~~~~~~~    ^

@flavorjones @nobu @shyouhei

[shyouhei]

OK. TBH an enum needs explicit cast to int sounds esoteric to me though.

[flavorjones]

@byroot Thanks. I'm sure there are probably more instances. I only addressed the ones that a) show up in MakeMakefile.try_compile or b) in the C extensions I maintain.

[byroot]

I found this one because it was failing the regression test when running make on my machine. Not too sure why CI didn't catch it.