definition fixup

stepmania/src/RageDisplay.cpp

@@ -51,6 +51,7 @@ int RageDisplay::GetDPF() const { return g_iDPF; }
 static int			g_iFramesRenderedSinceLastCheck,
 					g_iVertsRenderedSinceLastCheck,
 					g_iDrawsSinceLastCheck;
+PWSWAPINTERVALEXTPROC GLExt::wglSwapIntervalEXT;
 
 void GetGLExtensions(set<string> &ext)
 {

stepmania/src/RageDisplayInternal.h

@@ -45,7 +45,7 @@ struct oglspecs_t {
 /* Extension functions we use.  Put these in a namespace instead of in oglspecs_t,
  * so they can be called like regular functions. */
 namespace GLExt {
-	PWSWAPINTERVALEXTPROC wglSwapIntervalEXT;
+	extern PWSWAPINTERVALEXTPROC wglSwapIntervalEXT;
 };
 using namespace GLExt;