Fix verifier_test build

Some people insist on running make without specifying any target
like recoveryimage or otapackage, which triggers a build of
verifier_test, so this fix will keep people from asking about
this build error anymore.

Change-Id: Ic955b7a3fb2599894e712fd8b418227da240d288

roots.h

@@ -23,6 +23,8 @@
 extern "C" {
 #endif
 
+typedef struct fstab_rec Volume;
+
 // Load and parse volume data from /etc/recovery.fstab.
 void load_volume_table();