windowscodecs: only declare jpeg debug channel if jpeg is supported

[Austin English]

Found when compiling on a box without jpeg support.

-- 
-Austin
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diff --git a/dlls/windowscodecs/jpegformat.c b/dlls/windowscodecs/jpegformat.c
index 789a60b..ab21fbf 100644
--- a/dlls/windowscodecs/jpegformat.c
+++ b/dlls/windowscodecs/jpegformat.c
@@ -55,9 +55,9 @@
 #include "wine/library.h"
 
 WINE_DEFAULT_DEBUG_CHANNEL(wincodecs);
-WINE_DECLARE_DEBUG_CHANNEL(jpeg);
 
 #ifdef SONAME_LIBJPEG
+WINE_DECLARE_DEBUG_CHANNEL(jpeg);
 
 static void *libjpeg_handle;