Attempt to fix Windows link error for LLProcess::BasePipe::npos.

indra/llcommon/llprocess.h

@@ -291,7 +291,7 @@ class LL_COMMON_API LLProcess: public boost::noncopyable
 	std::string getPipeName(FILESLOT);
 
 	/// base of ReadPipe, WritePipe
-	class BasePipe
+	class LL_COMMON_API BasePipe
 	{
 	public:
 		virtual ~BasePipe() = 0;