lightnvm: make nvm_free static

Prefix the nvm_free static function with a missing static keyword.
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/lightnvm/core.c

@@ -999,7 +999,7 @@ static int nvm_core_init(struct nvm_dev *dev)
 	return ret;
 }
 
-void nvm_free(struct nvm_dev *dev)
+static void nvm_free(struct nvm_dev *dev)
 {
 	if (!dev)
 		return;