fix decl of ruby_single_main_ractor

On windows, MJIT doesn't work without this patch because of
the declaration of ruby_single_main_ractor. This patch fix this
issue and move the definition of it from ractor.c to vm.c to locate
near place of ruby_current_vm_ptr.

ractor.c

@@ -30,13 +30,6 @@ rb_ractor_error_class(void)
     return rb_eRactorError;
 }
 
-RUBY_SYMBOL_EXPORT_BEGIN
-
-// to share with MJIT
-rb_ractor_t *ruby_single_main_ractor;
-
-RUBY_SYMBOL_EXPORT_END
-
 static void vm_ractor_blocking_cnt_inc(rb_vm_t *vm, rb_ractor_t *r, const char *file, int line);
 
 static void

vm.c

@@ -378,6 +378,7 @@ VALUE rb_block_param_proxy;
 #define ruby_vm_redefined_flag GET_VM()->redefined_flag
 VALUE ruby_vm_const_missing_count = 0;
 rb_vm_t *ruby_current_vm_ptr = NULL;
+rb_ractor_t *ruby_single_main_ractor;
 
 #ifdef RB_THREAD_LOCAL_SPECIFIER
 RB_THREAD_LOCAL_SPECIFIER rb_execution_context_t *ruby_current_ec;

vm_core.h

@@ -1733,6 +1733,7 @@ rb_execution_context_t *rb_vm_main_ractor_ec(rb_vm_t *vm); // ractor.c
 #if RUBY_VM_THREAD_MODEL == 2
 RUBY_SYMBOL_EXPORT_BEGIN
 
+RUBY_EXTERN struct rb_ractor_struct *ruby_single_main_ractor; // ractor.c
 RUBY_EXTERN rb_vm_t *ruby_current_vm_ptr;
 RUBY_EXTERN rb_event_flag_t ruby_vm_event_flags;
 RUBY_EXTERN rb_event_flag_t ruby_vm_event_enabled_global_flags;
@@ -1799,8 +1800,6 @@ rb_current_thread(void)
     return rb_ec_thread_ptr(ec);
 }
 
-extern struct rb_ractor_struct *ruby_single_main_ractor; // ractor.c
-
 static inline rb_ractor_t *
 rb_current_ractor(void)
 {

vm_sync.h

@@ -31,7 +31,7 @@ void rb_vm_barrier(void);
 #include "vm_core.h"
 #endif
 
-extern struct rb_ractor_struct *ruby_single_main_ractor; // ractor.c
+RUBY_EXTERN struct rb_ractor_struct *ruby_single_main_ractor; // ractor.c
 
 static inline bool
 rb_multi_ractor_p(void)