tests: Fix typo in declspec keyword

This fixes the following errors with mingw toolchain.
OpenXR-SDK-Source/src/tests/hello_xr/main.cpp:17:10: error: expected constructor, destructor, or type conversion before '(' token
   17 | _declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
      |          ^
OpenXR-SDK-Source/src/tests/hello_xr/main.cpp:19:10: error: expected constructor, destructor, or type conversion before '(' token
   19 | _declspec(dllexport) DWORD AmdPowerXpressRequestHighPerformance = 0x00000001;
      |          ^

Author: Biswa96

src/tests/hello_xr/main.cpp

@@ -14,9 +14,9 @@
 // Favor the high performance NVIDIA or AMD GPUs
 extern "C" {
 // http://developer.download.nvidia.com/devzone/devcenter/gamegraphics/files/OptimusRenderingPolicies.pdf
-_declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
+__declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
 // https://gpuopen.com/learn/amdpowerxpressrequesthighperformance/
-_declspec(dllexport) DWORD AmdPowerXpressRequestHighPerformance = 0x00000001;
+__declspec(dllexport) DWORD AmdPowerXpressRequestHighPerformance = 0x00000001;
 }
 #endif  // defined(_WIN32)