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.c

@@ -81,6 +81,11 @@ extern int wScreenCount;
 extern int wXkbSupported;
 #endif
 
+#ifdef HAVE_XRANDR
+Bool has_randr;
+int randr_event_base;
+#endif
+
 extern WDDomain *WDWindowMaker;
 
 /**** Local ****/