Add noexcept qualifications in src/common

Apparently this can actually make a difference in some cases, and the
C++ Core Guideliens recommend doing this on all default constructors,
destructors, and all functions that can not throw (and thus also don't
allocate).

src/common/plugins.h

@@ -60,5 +60,5 @@ void serialize(S& s, PluginType& plugin_type) {
  */
 LibArchitecture find_dll_architecture(const boost::filesystem::path&);
 
-PluginType plugin_type_from_string(const std::string& plugin_type);
+PluginType plugin_type_from_string(const std::string& plugin_type) noexcept;
 std::string plugin_type_to_string(const PluginType& plugin_type);