Add back two declarations needed for test-gen

These were removed in diffblue#4216.

This is temporary and these two declarations will be removed once
https://github.com/diffblue/test-gen/tree/svorenova/goto-checker
is merged.

Author: Joel Allred

src/cbmc/bmc.h

@@ -193,6 +193,10 @@ class bmct:public safety_checkert
 
   friend class bmc_all_propertiest;
 
+  // \todo remove these two declarations once test-gen dependency is fixed
+  bool cover(const goto_functionst &goto_functions);
+  friend class bmc_covert;
+
 private:
   /// \brief Class-specific symbolic execution
   ///