vty: suppress warnings about len being set but not used

This commit fixes the following warning seen with CC=clang:

utils.c:376:6: warning: variable 'len' set but not used [-Wunused-but-set-variable]
        int len = 0, offset = 0, ret, rem;

... and finally allows to build libosmocore with --enable-werror.

Change-Id: I0040ef20ba3fc53ee7ccefc4885170f333f80566
---
M src/vty/utils.c
1 file changed, 17 insertions(+), 0 deletions(-)

diff --git a/src/vty/utils.c b/src/vty/utils.c
index a651515..0f5a34e 100644
--- a/src/vty/utils.c
+++ b/src/vty/utils.c
@@ -415,6 +415,7 @@
 	if (ret < 0)
 		goto err;
 	OSMO_SNPRINTF_RET(ret, rem, offset, len);
+	(void)len; /* suppress warnings about len being set but not used */
 err:
 	str[size-1] = '\0';
 	return str;

