Fixed improper variables definition in header file

Some header were creating variable, this is a bad practice which
is likely to not behave as expected. This creates one distinct
variable in each object file that used the header, and:

 - on well behaved compiler, this ends up in a link error (see
commit 39fdb451ba for an example)

 - on bad behaving compiler, this can be linked as multiple local
variable, thus having strange effects when running program

 - on insouciant compiler (who said gcc?) the variables are
silently merged, hiding portability issues

src/screen.h

@@ -36,8 +36,8 @@
 #define WTB_MENU 6
 
 #ifdef HAVE_XRANDR
-Bool has_randr;
-int randr_event_base;
+extern Bool has_randr;
+extern int randr_event_base;
 #endif
 
 typedef struct {