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/image_manager_main.cpp b/image_manager_main.cpp
index af711a7..867a960 100644
--- a/image_manager_main.cpp
+++ b/image_manager_main.cpp
@@ -8,7 +8,7 @@
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/bus.hpp>
 
-int main(int argc, char* argv[])
+int main()
 {
     using namespace phosphor::software::manager;
     auto bus = sdbusplus::bus::new_default();