target/ppc: use g_free() in test_opcode_table()

table[i] is allocated in create_new_table() using g_new().

Use g_free(table[i]) instead of free(table[i]) to comply with QEMU low
level memory management guidelines.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
[Mjt: minor commit comment tweak]