ident.c: replace fprintf with fputs to suppress compiler warning

Compiling today's pu gave
    ...
    CC ident.o
    CC levenshtein.o
ident.c: In function 'fmt_ident':
ident.c:206: warning: format not a string literal and no format arguments
    CC list-objects.o
    ...

This warning seems to have appeared first in 18e95f279e (ident.c:
remove unused variables) which removed additional fprintf arguments.

Suppress this warning by using fputs instead of fprintf.

Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ident.c

@@ -203,7 +203,7 @@ const char *fmt_ident(const char *name, const char *email,
 
 		if ((warn_on_no_name || error_on_no_name) &&
 		    name == git_default_name && env_hint) {
-			fprintf(stderr, env_hint);
+			fputs(env_hint, stderr);
 			env_hint = NULL; /* warn only once */
 		}
 		if (error_on_no_name)