fix: cert-err58 warning in GTK client

[ckerr]

Fix these cert-err58-cpp warnings:

[95/150] Building CXX object gtk/CMakeFiles/transmission-gtk.dir/SystemTrayIcon.cc.o
/home/charles/src/transmission/gtk/SystemTrayIcon.cc:58:12: warning: initialization of 'TrayIconName' with static storage duration may throw an exception that cannot be caught [cert-err58-cpp]
auto const TrayIconName = Glib::ustring("transmission-tray-icon"s);
^
/usr/include/glibmm-2.4/glibmm/ustring.h:403:14: note: possibly throwing constructor declared here
GLIBMM_API ustring(std::string&& src);
^
/home/charles/src/transmission/gtk/SystemTrayIcon.cc:59:12: warning: initialization of 'AppIconName' with static storage duration may throw an exception that cannot be caught [cert-err58-cpp]
auto const AppIconName = Glib::ustring("transmission"s);
^
/usr/include/glibmm-2.4/glibmm/ustring.h:403:14: note: possibly throwing constructor declared here
GLIBMM_API ustring(std::string&& src);
^
/home/charles/src/transmission/gtk/SystemTrayIcon.cc:63:12: warning: initialization of 'AppName' with static storage duration may throw an exception that cannot be caught [cert-err58-cpp]
auto const AppName = Glib::ustring("transmission-gtk"s);
^
/usr/include/glibmm-2.4/glibmm/ustring.h:403:14: note: possibly throwing constructor declared here
GLIBMM_API ustring(std::string&& src);
^