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/viml/parser/expressions.c

@@ -59,7 +59,6 @@
 #include "nvim/assert_defs.h"
 #include "nvim/charset.h"
 #include "nvim/eval.h"
-#include "nvim/func_attr.h"
 #include "nvim/gettext.h"
 #include "nvim/keycodes.h"
 #include "nvim/macros_defs.h"