vtysh: allow --with-libpam to build with --enable-werror

The function vtysh_pam fails the build with --enable-werror enabled
because it is a static function not declared as such

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/vtysh/vtysh_user.c b/vtysh/vtysh_user.c
index e1c611c..7e10d68 100644
--- a/vtysh/vtysh_user.c
+++ b/vtysh/vtysh_user.c
@@ -46,7 +46,7 @@
   NULL
 };
 
-int
+static int
 vtysh_pam (const char *user)
 {
   int ret;