This is a wrapper around g_private_set() which allocates the desired
amount of memory for the caller and calls g_private_set() on it.

This is intended to make it easier to suppress Valgrind warnings about
leaked memory, since g_private_set() is typically used to make one-time
per-thread allocations. We can now just add a blanket suppression rule
for any allocations inside g_private_set_alloc0().

Signed-off-by: Philip Withnall <withnall@endlessm.com>