Always #include <ruby.h> before any system header.

* Fixes #88,
  the issue explains it in details.
* `#include "extconf.h"` is redundant and a code smell for missing ruby.h,
  ruby/internal/config.h already does `#include RUBY_EXTCONF_H`.

ext/io/event/event.h

@@ -22,8 +22,6 @@
 
 #include <ruby.h>
 
-#include "extconf.h"
-
 void Init_IO_Event(void);
 
 #ifdef HAVE_LIBURING_H

ext/io/event/interrupt.h

@@ -20,7 +20,7 @@
 
 #pragma once
 
-#include "extconf.h"
+#include <ruby.h>
 
 #ifdef HAVE_SYS_EVENTFD_H
 struct IO_Event_Interrupt {

ext/io/event/selector/array.h

@@ -3,6 +3,7 @@
 
 // Provides a simple implementation of unique pointers to elements of the given size.
 
+#include <ruby.h>
 #include <stdlib.h>
 #include <errno.h>
 #include <assert.h>

ext/io/event/selector/list.h

@@ -1,6 +1,7 @@
 // Released under the MIT License.
 // Copyright, 2023, by Samuel Williams.
 
+#include <ruby.h>
 #include <stdio.h>
 #include <assert.h>
 

ext/io/event/selector/pidfd.c

@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#include <ruby.h>
+
 #include <sys/types.h>
 #include <sys/syscall.h>
 #include <unistd.h>