ci/build-all-defconfigs.sh: fix libeatmydata.so handling

The test for 'libeatmydata.so' always succeeds since 'test -f' is always
true for null strings. Fix that.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/ci/build-all-defconfigs.sh b/ci/build-all-defconfigs.sh
index 096ffbe..f60217e 100755
--- a/ci/build-all-defconfigs.sh
+++ b/ci/build-all-defconfigs.sh
@@ -68,7 +68,7 @@
 	export BR2_DL_DIR=${DL_DIR}
 fi
 
-if [ -f $(ldconfig -p | grep libeatmydata.so | tr ' ' '\n' | grep /|head -n1) ]; then
+if [ -f "$(ldconfig -p | grep libeatmydata.so | tr ' ' '\n' | grep /|head -n1)" ]; then
     export LD_PRELOAD=${LD_PRELOAD:+"$LD_PRELOAD "}libeatmydata.so
 fi