Fix KWS NODE_MEMREQ instance header underallocation

[goyalpalak18]

While looking at the recent ABF memory patch (commit edc44cf), I noticed the exact same under-allocation bug exists in the KWS component.

The Bug

In src/ee_kws.c, NODE_MEMREQ allocates memory using a magic number: sizeof(mfcc_instance_t) + 8. The + 8 was probably meant to cover two 32-bit pointers, but it ignores the chunk_idx field entirely and breaks on 64-bit platforms (where pointers are 8 bytes). There's literally a /* TODO : justift this */ comment sitting right next to it.

Because of this, the kws_instance_t struct overflows its heap block. This eats into the 12-byte CMSIS vectorized-read safety padding, leading to undefined behavior. On 64-bit RISC-V evaluation targets, this overflow can silently corrupt heap metadata or cause sliding window drifts that completely invalidate benchmark scores.

The Fix

I mirrored the ABF fix by replacing the manual calculation with a standard sizeof(kws_instance_t). Since the struct already includes mfcc_instance_t as its first member, this naturally accounts for all pointers, chunk_idx, and any compiler-inserted alignment padding across all architectures and pointer widths.

// Before:
uint32_t size = (3 * 4) // See note above
                + sizeof(mfcc_instance_t)
                + 8; /* TODO : justift this */

// After:
uint32_t size = (3 * 4) // See note above
                + sizeof(kws_instance_t);

This restores the full CMSIS 12-byte guard padding, eliminates the undefined behavior risk, and brings KWS up to the same correctness baseline as ABF.

[goyalpalak18]

Hey @llefaucheur @joseph-yiu, could you take a look at this when you have a chance? Thanks!