obmcutil: Quote variable in non-zero test to avoid always true result

Currently adding `--wait` invokes verbose output even if `--verbose` is
not supplied.

Change-Id: Ib14d0cc7e6f0bf31d147537b58f1a65b7d99450a
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/obmcutil b/obmcutil
index d8463d5..59a1801 100644
--- a/obmcutil
+++ b/obmcutil
@@ -53,7 +53,7 @@
     local cmd="$@"
     local verbose_child=
 
-    if [ -n $G_VERBOSE ]; then
+    if [ -n "$G_VERBOSE" ]; then
         journalctl -f &
         verbose_child=$!
     fi