shadow: fix warning

Fixes:

	gportay@archlinux ~/src/iamroot $ make
	(...)
	cc -g -Werror -DNVERBOSE -fPIC -Wall -Wextra -D_GNU_SOURCE -DVERSION=23 -DPREFIX=/usr/local  -c -o shadow.o /home/gportay/src/iamroot/shadow.c
	/home/gportay/src/iamroot/shadow.c: In function ‘fgetspent’:
	/home/gportay/src/iamroot/shadow.c:55:19: error: unused variable ‘fd’ [-Werror=unused-variable]
	   55 |         const int fd = fileno(f);
	      |                   ^~
	/home/gportay/src/iamroot/shadow.c: In function ‘putspent’:
	/home/gportay/src/iamroot/shadow.c:91:19: error: unused variable ‘fd’ [-Werror=unused-variable]
	   91 |         const int fd = fileno(f);
	      |                   ^~
	cc1: all warnings being treated as errors

shadow.c

@@ -54,6 +54,7 @@ struct spwd *fgetspent(FILE *f)
 {
 	const int fd = fileno(f);
 	struct spwd *ret;
+	(void)fd;
 
 	ret = _fgetspent(f);
 
@@ -90,6 +91,7 @@ int putspent(const struct spwd *sp, FILE *f)
 {
 	const int fd = fileno(f);
 	int ret;
+	(void)fd;
 
 	ret = _putspent(sp, f);