Fix major incorrect usage of mrb class definitions causing segfaults

Makefile

@@ -24,7 +24,7 @@ CFLAGS_DEBUG := \
 	-std=c++20 -Wall -Wextra \
 	-O0 -g -fno-omit-frame-pointer \
 	-Wno-unused-command-line-argument \
-	-I./include \
+	-I./include -I./libs/mruby/include \
 	$(SDL_CFLAGS)
 
 CFLAGS_RELEASE := \
@@ -32,7 +32,7 @@ CFLAGS_RELEASE := \
 	-flto=thin \
 	-fno-rtti -fomit-frame-pointer -DNDEBUG -s \
 	-Wno-unused-command-line-argument \
-	-I./include \
+	-I./include -I./libs/mruby/include \
 	$(SDL_CFLAGS)
 
 PCH_CFLAGS_DEBUG := $(CFLAGS_DEBUG) -x c++-header