maint: fix recent syntax-check failures

* cfg.mk (exclude_file_name_regexp--sc_system_h_headers):
Add chown-core.h to the regexp, to better decouple from system.h.
* src/env.c: Remove minmax.h include already included in system.h.
* src/libstdbuf.c: Likewise.
* src/prog-fprintf.h: Remove doubled semicolon.

src/env.c

@@ -27,7 +27,6 @@
 #include "die.h"
 #include "error.h"
 #include "idx.h"
-#include "minmax.h"
 #include "operand2sig.h"
 #include "quote.h"
 #include "sig2str.h"

src/libstdbuf.c

@@ -20,7 +20,6 @@
 #include <stdio.h>
 #include <stdint.h>
 #include "system.h"
-#include "minmax.h"
 
 /* Deactivate config.h's "rpl_"-prefixed definitions, since we don't
    link gnulib here, and the replacements aren't needed.  */

src/prog-fprintf.h

@@ -20,6 +20,6 @@
 # include <stdio.h>
 
 extern void prog_fprintf (FILE *fp, char const *fmt, ...)
-  _GL_ATTRIBUTE_FORMAT ((__printf__, 2, 3)) _GL_ATTRIBUTE_NONNULL ((1, 2));;
+  _GL_ATTRIBUTE_FORMAT ((__printf__, 2, 3)) _GL_ATTRIBUTE_NONNULL ((1, 2));
 
 #endif