maint: include libstdbuf.c in extraneous headers check

* cfg.mk (sc_system_h_headers): Don't exclude libstdbuf.c.
* src/libstdbuf.c: Remove headers already included in system.h.

cfg.mk

@@ -603,7 +603,7 @@ exclude_file_name_regexp--sc_bindtextdomain = \
   ^(gl/.*|lib/euidaccess-stat|src/make-prime-list)\.c$$
 exclude_file_name_regexp--sc_trailing_blank = ^(tests/pr/|man/help2man)
 exclude_file_name_regexp--sc_system_h_headers = \
-  ^src/((system|copy)\.h|libstdbuf\.c|make-prime-list\.c)$$
+  ^src/((system|copy)\.h|make-prime-list\.c)$$
 
 _src = (false|lbracket|ls-(dir|ls|vdir)|tac-pipe|uname-(arch|uname))
 exclude_file_name_regexp--sc_require_config_h_first = \

src/libstdbuf.c

@@ -18,9 +18,7 @@
 
 #include <config.h>
 #include <stdio.h>
-#include <stdlib.h>
 #include "system.h"
-#include "verify.h"
 
 /* Note currently for glibc (2.3.5) the following call does not change
    the buffer size, and more problematically does not give any indication