boot: avoid 32-bit calculation for a 64-bit lvalue

boot: avoid 32-bit calculation for a 64-bit lvalue

Coverity CID#1399116:
> Potentially overflowing expression
> gpt_header_buffer.gpt_header.SizeOfPartitionEntry * gpt_header_buffer.gpt_header.NumberOfPartitionEntries
> with type unsigned int (32 bits, unsigned) is evaluated using 32-bit
> arithmetic, and then used in a context that expects an expression of type
> UINTN (64 bits, unsigned).

Let's import the ALIGN_TO macro to sd-boot and use it to avoid the issue.