DSPSpy: Fix build failures

[ghost]

Remove dangling #endif include guard left by d203804

main_spy.cpp
In file included from dolphin/Source/DSPSpy/./main_spy.cpp:47:0:
dolphin/Source/DSPSpy/./dsp_interface.h:36:2: error: #endif without #if
 #endif  // _DSP_INTERFACE_H
  ^

Enable C++0x support for nullptr introduced by d802d39
-std=c++0x was used instead of -std=c++11 to support older versions of DevkitPPC (r23-26) that use GCC 4.6.x

main_spy.cpp
dolphin/Source/DSPSpy/./main_spy.cpp:54:1: warning: identifier 'nullptr' is a keyword in C++11 [-Wc++0x-compat]
 static void *xfb = nullptr;
 ^
dolphin/Source/DSPSpy/./main_spy.cpp:54:20: error: 'nullptr' was not declared in this scope
 static void *xfb = nullptr;
                    ^
dolphin/Source/DSPSpy/./main_spy.cpp: In function 'void InitGeneral()':
dolphin/Source/DSPSpy/./main_spy.cpp:529:33: error: 'nullptr' was not declared in this scope
  rmode = VIDEO_GetPreferredMode(nullptr);
                                 ^

Tested and built using DevkitPPC r27

[skidau]

@dolphin-emu-bot rebuild