adjustments to support gcc10

Gcc changed its default behaviour in version 10.
-fno-common is now used by default instead of -fcommon.
Therefore the usage of the extern keyword is required.

ueberzug/X/Xshm.h

@@ -3,5 +3,5 @@
 #include "python.h"
 
 
-PyTypeObject ImageType;
+extern PyTypeObject ImageType;
 #endif

ueberzug/X/display.h

@@ -24,6 +24,6 @@ typedef struct {
     Atom wm_locale_name;
     Atom wm_normal_hints;
 } DisplayObject;
-PyTypeObject DisplayType;
+extern PyTypeObject DisplayType;
 
 #endif

ueberzug/X/window.h

@@ -3,5 +3,5 @@
 #include "python.h"
 
 
-PyTypeObject WindowType;
+extern PyTypeObject WindowType;
 #endif