build: don't define FUNC_ATTR_* as empty in headers (#26317)

FUNC_ATTR_* should only be used in .c files with generated headers.
Defining FUNC_ATTR_* as empty in headers causes misuses of them to be
silently ignored. Instead don't define them by default, and only define
them as empty after a .c file has included its generated header.

src/nvim/msgpack_rpc/channel.c

@@ -20,7 +20,6 @@
 #include "nvim/event/rstream.h"
 #include "nvim/event/stream.h"
 #include "nvim/event/wstream.h"
-#include "nvim/func_attr.h"
 #include "nvim/globals.h"
 #include "nvim/log.h"
 #include "nvim/main.h"

src/nvim/msgpack_rpc/helpers.c

@@ -10,7 +10,6 @@
 #include "msgpack/pack.h"
 #include "nvim/api/private/helpers.h"
 #include "nvim/assert_defs.h"
-#include "nvim/func_attr.h"
 #include "nvim/memory.h"
 #include "nvim/msgpack_rpc/helpers.h"
 #include "nvim/types_defs.h"

src/nvim/msgpack_rpc/server.c

@@ -7,7 +7,6 @@
 #include "nvim/channel.h"
 #include "nvim/eval.h"
 #include "nvim/event/socket.h"
-#include "nvim/func_attr.h"
 #include "nvim/garray.h"
 #include "nvim/log.h"
 #include "nvim/main.h"