Let WinRT dll Build

WinRT dll should also use __declspec to build.

include/bgfx/c99/bgfx.h

@@ -34,7 +34,7 @@
 #    define BGFX_SHARED_LIB_USE 0
 #endif // BGFX_SHARED_LIB_USE
 
-#if BX_PLATFORM_WINDOWS
+#if BX_PLATFORM_WINDOWS||BX_PLATFORM_WINRT
 #   define BGFX_SYMBOL_EXPORT __declspec(dllexport)
 #   define BGFX_SYMBOL_IMPORT __declspec(dllimport)
 #else