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"

src/nvim/viml/parser/parser.c

@@ -5,6 +5,7 @@
 #endif
 
 void parser_simple_get_line(void *cookie, ParserLine *ret_pline)
+  FUNC_ATTR_NONNULL_ALL
 {
   ParserLine **plines_p = (ParserLine **)cookie;
   *ret_pline = **plines_p;