Fix up SDL2 include paths

Using "SDL2/SDL_xxx.h" instead of "SDL_xxx.h" caused
the include paths provided by pkg-config to be ignored,
and headers from a standard include path to be used instead.

binding-mruby/mrb-ext/file.cpp

@@ -26,7 +26,7 @@
 #include "mruby/array.h"
 #include "mruby/class.h"
 
-#include "SDL2/SDL_rwops.h"
+#include "SDL_rwops.h"
 
 #include <stdio.h>
 #include <errno.h>

binding-mruby/mrb-ext/file.h

@@ -24,7 +24,7 @@
 
 #include "../binding-util.h"
 
-#include "SDL2/SDL_rwops.h"
+#include "SDL_rwops.h"
 
 struct FileImpl
 {

binding-mruby/mrb-ext/kernel.cpp

@@ -26,7 +26,7 @@
 #include <stdlib.h>
 #include <sys/time.h>
 
-#include "SDL2/SDL_messagebox.h"
+#include "SDL_messagebox.h"
 
 #include "../binding-util.h"
 #include "marshal.h"

binding-mruby/mrb-ext/marshal.cpp

@@ -38,7 +38,7 @@
 #include "rwmem.h"
 #include "exception.h"
 
-#include "SDL2/SDL_timer.h"
+#include "SDL_timer.h"
 
 #include <QVector>
 #include <QHash>

binding-mruby/mrb-ext/marshal.h

@@ -23,7 +23,7 @@
 #define MARSHAL_H
 
 #include "mruby.h"
-#include "SDL2/SDL_rwops.h"
+#include "SDL_rwops.h"
 
 void marshalDumpInt(mrb_state *, SDL_RWops *, mrb_value);
 mrb_value marshalLoadInt(mrb_state *, SDL_RWops *);

binding-mruby/mrb-ext/rwmem.cpp

@@ -21,7 +21,7 @@
 
 #include "rwmem.h"
 
-#include "SDL2/SDL_rwops.h"
+#include "SDL_rwops.h"
 #include <QVector>
 
 typedef QVector<char> ByteVec;