openpower-pels: clean up various compile warnings

Compile warnings observed when compiling parts of the
openpower-pels (or corresponding tests) under stricter
compiler warning flags of Meson.

Issues fixed:
    - many unused parameters
    - invalid case fall-through
    - excess semi-colons
    - incorrect 'const' on return-by-value type
    - removal of variable length array in test case
    - uncaught return from 'system' call in test case

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8af69184042cf8661d1307a02ecf3afcab4724a1
diff --git a/extensions/openpower-pels/src.cpp b/extensions/openpower-pels/src.cpp
index 0bbc61f..9bc3327 100644
--- a/extensions/openpower-pels/src.cpp
+++ b/extensions/openpower-pels/src.cpp
@@ -636,7 +636,8 @@
 }
 
 std::optional<std::string> SRC::getJSON(message::Registry& registry,
-                                        const std::vector<std::string>& plugins,
+                                        const std::vector<std::string>& plugins
+                                        [[maybe_unused]],
                                         uint8_t creatorID) const
 {
     std::string ps;