Undefine allocation function for C extension class

* Undefine allocation function for C extension class

Since Ruby 3.2 a new warning is printed when a Ruby class created in a C
extension does not specify an allocate function or undefine it.

```
/gem/lib/appsignal/transaction.rb:98: warning: undefining the allocator of T_DATA class Appsignal::Extension::Transaction
/gem/lib/appsignal/utils/data.rb:19: warning: undefining the allocator of T_DATA class Appsignal::Extension::Data
```

From my understanding, we only need to define an allocate function if
the class uses a C struct and stores any values on it. Our classes don't
do that in C, that's done in our Rust extension.

Closes #909

## Resources

https://bugs.ruby-lang.org/issues/18007
https://github.com/ruby/ruby/blob/6963f8f743b42f9004a0879cd66c550f18987352/doc/extension.rdoc#label-Write+the+C+Code
https://ruby-doc.org/core-3.1.1/doc/extension_rdoc.html#label-C+struct+to+Ruby+object

rails-sqlserver/tiny_tds#515
https://groups.google.com/g/sequel-talk/c/K0J80s4wGJU/m/BT-6FFhrAgAJ

## Other MR doing similar change
vmg/redcarpet#721
appsignal/appsignal-ruby#917

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## 0.8.0 [2023-11-??]
 
+* Remove Warning 'warning: undefining the allocator of T_DATA class FileMagic'
+
 * Require ruby version 3.0.0 or later.
 
 * Separate error messages for missing header and library in extconf.rb

ext/filemagic/filemagic.c

@@ -210,6 +210,7 @@ void
 Init_ruby_filemagic() {
   char version[16] = "0";
   cFileMagic = rb_define_class("FileMagic", rb_cObject);
+  rb_undef_alloc_func(cFileMagic);
 
 #if defined(FILE_VERSION_MAJOR)
   RB_MAGIC_SET_VERSION(FILE_VERSION_MAJOR, patchlevel)
@@ -241,6 +242,7 @@ Init_ruby_filemagic() {
   rb_alias(cFileMagic, rb_intern("valid?"), rb_intern("check"));
 
   rb_FileMagicError = rb_define_class_under(cFileMagic, "FileMagicError", rb_eStandardError);
+  rb_undef_alloc_func(rb_FileMagicError);
 
   /*======= BEGIN CONSTANTS =======*/
 #ifdef MAGIC_NONE

test/mahoro.c

@@ -159,7 +159,9 @@ mahoro_load(self, path)
 void Init_mahoro(void)
 {
 	cMahoro      = rb_define_class("Mahoro", rb_cObject);
+	rb_undef_alloc_func(cMahoro);
 	eMahoroError = rb_define_class_under(cMahoro, "Error", rb_eStandardError);
+	rb_undef_alloc_func(eMahoroError);
 
 	rb_const_set(cMahoro, rb_intern("NONE"), INT2FIX(MAGIC_NONE));
 	rb_const_set(cMahoro, rb_intern("DEBUG"), INT2FIX(MAGIC_DEBUG));