Suppress leaks in g_signal_new() properly. (LP: #1102822)

The previous attempt (LP: #1097649) had no effect because valgrind requires
suppression rules match the function at the top of the stack explicitly
(calloc/malloc...). So that's what I've added here, and now it works.
. Fixes: https://bugs.launchpad.net/bugs/1102822.

Approved by Sam Spilsbury, PS Jenkins bot.