Add pragma in first Echidna exercise

The first exercise rests on the fact that
the developer must compile with a version
lower than 0.8.0 where overflow/underflow
errors are not an issue anymore.

Author: 0xmichalis

program-analysis/echidna/exercises/exercise1/solution.sol

@@ -1,3 +1,6 @@
+//SPDX-License-Identifier: AGPL-3.0-or-later
+pragma solidity <0.8.0;
+
 import "token.sol";
 
 /// @dev to run: $ echidna-test solution.sol

program-analysis/echidna/exercises/exercise1/template.sol

@@ -1,3 +1,6 @@
+//SPDX-License-Identifier: AGPL-3.0-or-later
+pragma solidity <0.8.0;
+
 import "token.sol";
 
 contract TestToken is Token {

program-analysis/echidna/exercises/exercise1/token.sol

@@ -1,4 +1,5 @@
-
+//SPDX-License-Identifier: AGPL-3.0-or-later
+pragma solidity <0.8.0;
 
 contract Ownership{