meson: Fix compile errors

Meson runs stricter compiler checks. Fix the following errors in
preparation to moving to meson:

- Unusued parameters. Ex:
../download_manager_main.cpp:7:14: error: unused parameter ‘argc’ [-Werror=unused-parameter]
../static/item_updater_helper.cpp:16:42: error: unused parameter ‘entryId’ [-Werror=unused-parameter]
../watch.cpp:77:38: error: unused parameter ‘s’ [-Werror=unused-parameter]
[test/utest.cpp:166]: (style) Variable 'pubKeyConfFile' is assigned a value that is never used.

- Add destructor and initialize variable in init list:
../flash.hpp:18:7: error: ‘class phosphor::software::updater::Flash’ has virtual functions and accessible non-virtual destructor [-Werror=non-virtual-dtor]
[version.hpp:81]: callback is assigned in constructor body. Consider performing initialization in initialization list

Tested: Verified code update on witherspoon still worked.

Change-Id: Ib7a0e0e11d6bfe6eeeb3f2ea4bfefd43505d639e
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/flash.hpp b/flash.hpp
index 5069680..c276017 100644
--- a/flash.hpp
+++ b/flash.hpp
@@ -18,6 +18,9 @@
 class Flash
 {
   public:
+    /* Destructor */
+    virtual ~Flash() = default;
+
     /**
      * @brief Writes the image file(s) to flash
      */