nvim: Correctly setup global channels

As gcc10 uses -fno-common by default, global variables declared with the
same name more than once is not allowed anymore revealing this issue.

We need to define it as extern to access it.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1799680

src/nvim/channel.c

@@ -19,7 +19,6 @@
 #include "nvim/ascii.h"
 
 static bool did_stdio = false;
-PMap(uint64_t) *channels = NULL;
 
 /// next free id for a job or rpc channel
 /// 1 is reserved for stdio channel

src/nvim/channel.h

@@ -85,7 +85,7 @@ struct Channel {
   bool callback_scheduled;
 };
 
-EXTERN PMap(uint64_t) *channels;
+EXTERN PMap(uint64_t) *channels INIT(= NULL);
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "channel.h.generated.h"

src/nvim/main.c

@@ -10,6 +10,7 @@
 #include <msgpack.h>
 
 #include "nvim/ascii.h"
+#include "nvim/channel.h"
 #include "nvim/vim.h"
 #include "nvim/main.h"
 #include "nvim/aucmd.h"