util/ralloc: fix ralloc alignment.

util/ralloc: fix ralloc alignment.

On some malloc implementation, malloc doesn't always align to 16
bytes even on 64 bits system. To make sure ralloc_header always
starts at the wanted alignment, just force the size to be aligned at
the alignment of ralloc_header. This fixes crashed on instruction
like "movaps %xmm0,0x10(%rax)" which requires aligned memory access.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6314>