plug-ins: Fix unused variable warning in file-xpm

In file-xpm, use_x_api is only used when
GDK_WINDOWING_X11 is defined. However, it was
included as a general variable so it threw an unused variable
warning when compiled on other systems.
This patch moves the variable declaration inside the first
#ifdef GDK_WINDOWING_X11 block of the parse_colors ()
function.

plug-ins/common/file-xpm.c

@@ -418,10 +418,10 @@ parse_colors (XpmImage *xpm_image)
 #ifdef GDK_WINDOWING_X11
   Display  *display;
   Colormap  colormap;
+  gboolean  use_x_api = FALSE;
 #endif
   gint      i, j;
   guchar   *cmap;
-  gboolean use_x_api = FALSE;;
 
 #ifdef GDK_WINDOWING_X11
   use_x_api = (gdk_display_get_default () &&