Fix string_printf linking

Author: Mr-Wiseguy

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "mm-decomp"]
 	path = mm-decomp
 	url = https://github.com/zeldaret/mm
+[submodule "lib/rapidjson"]
+	path = lib/rapidjson
+	url = https://github.com/Tencent/rapidjson

lib/rapidjson

@@ -1,6 +1,8 @@
 #include "string_printf.hpp"
 #include "stdarg.h"
-#include "PR/xstdio.h"
+extern "C" {
+    #include "PR/xstdio.h"
+}
 
 void* snprintf_pfn(void* dst, const char* fmt, size_t size) {
     std::string* str = reinterpret_cast<std::string*>(dst);