stpcpy.3: Add missing pieces/fix various problems in example program

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/stpcpy.3

@@ -59,12 +59,16 @@ to concatenate \fBfoo\fP and
 .in +4n
 .nf
 
+#define _GNU_SOURCE
 #include <string.h>
+#include <stdio.h>
 
 int
 main(void)
 {
+    char buffer[20];
     char *to = buffer;
+
     to = stpcpy(to, "foo");
     to = stpcpy(to, "bar");
     printf("%s\\n", buffer);