Add dosfstools to files

dosfstools provides mkfs.vfat which is needed if n-cpu is
configured with 'config_drive_format=vfat'.

Change-Id: If1e1537a079e71847d91ae03ed0c18290a467c4e
Related-Bug: #1770640
diff --git a/files/debs/n-cpu b/files/debs/n-cpu
index d8bbf59..636644f 100644
--- a/files/debs/n-cpu
+++ b/files/debs/n-cpu
@@ -1,4 +1,5 @@
 cryptsetup
+dosfstools
 genisoimage
 gir1.2-libosinfo-1.0
 lvm2 # NOPRIME