group: fix warning

Fixes:

	-bash-5.2$ make
	(...)
	cc -Werror -DNVERBOSE -fPIC -Wall -Wextra -D_GNU_SOURCE -DVERSION=23 -DPREFIX=/usr/local  -c -o group.o /home/freebox/src/iamroot/group.c
	/home/freebox/src/iamroot/group.c:61:12: error: unused variable 'fd' [-Werror,-Wunused-variable]
	        const int fd = fileno(f);
	                  ^
	/home/freebox/src/iamroot/group.c:119:12: error: unused variable 'fd' [-Werror,-Wunused-variable]
	        const int fd = fileno(f);
	                  ^
	2 errors generated.

group.c

@@ -60,6 +60,7 @@ struct group *fgetgrent(FILE *f)
 {
 	const int fd = fileno(f);
 	struct group *ret;
+	(void)fd;
 
 	ret = _fgetgrent(f);
 
@@ -118,6 +119,7 @@ int putgrent(const struct group *gr, FILE *f)
 {
 	const int fd = fileno(f);
 	int ret;
+	(void)fd;
 
 	ret = _putgrent(gr, f);