hw/riscv/virt-acpi-build.c: fix leak in build_rhct()

hw/riscv/virt-acpi-build.c: fix leak in build_rhct()

The 'isa' char pointer isn't being freed after use.

Issue detected by Valgrind:

==38752== 128 bytes in 1 blocks are definitely lost in loss record 3,190 of 3,884
==38752==    at 0x484280F: malloc (vg_replace_malloc.c:442)
==38752==    by 0x5189619: g_malloc (gmem.c:130)
==38752==    by 0x51A5BF2: g_strconcat (gstrfuncs.c:628)
==38752==    by 0x6C1E3E: riscv_isa_string_ext (cpu.c:2321)
==38752==    by 0x6C1E3E: riscv_isa_string (cpu.c:2343)
==38752==    by 0x6BD2EA: build_rhct (virt-acpi-build.c:232)
==38752==    by 0x6BD2EA: virt_acpi_build (virt-acpi-build.c:556)
==38752==    by 0x6BDC86: virt_acpi_setup (virt-acpi-build.c:662)
==38752==    by 0x9C8DC6: notifier_list_notify (notify.c:39)
==38752==    by 0x4A595A: qdev_machine_creation_done (machine.c:1589)
==38752==    by 0x61E052: qemu_machine_creation_done (vl.c:2680)
==38752==    by 0x61E052: qmp_x_exit_preconfig.part.0 (vl.c:2709)
==38752==    by 0x6220C6: qmp_x_exit_preconfig (vl.c:2702)
==38752==    by 0x6220C6: qemu_init (vl.c:3758)
==38752==    by 0x425858: main (main.c:47)

Fixes: ebfd392893 ("hw/riscv/virt: virt-acpi-build.c: Add RHCT Table")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240122221529.86562-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>