modules/bluez4: fix duplicated symbol

Summary:
Global variable should be declared as extern to avoid symbol duplication.
Fix compiling of ebluez4 module on OS X.

Test Plan:
Since Enlightenment cannot (yet) be fully compiled on OS X, I cannot test
the real-life impact of this change.

Reviewers: zmike, raster

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2327

src/modules/bluez4/ebluez4.c

@@ -4,6 +4,8 @@
 #include "e_mod_main.h"
 #include "ebluez4.h"
 
+Context *ctxt = NULL;
+
 typedef struct _Pair_Cb
 {
    void (*cb)(void *, Eina_Bool, const char *);

src/modules/bluez4/ebluez4.h

@@ -70,7 +70,7 @@ typedef struct _Context
    Eina_List *adapters;
 } Context;
 
-Context *ctxt;
+extern Context *ctxt;
 
 void ebluez4_eldbus_init(void);
 void ebluez4_eldbus_shutdown(void);