nir/spirv: fix MSVC warning in vtn_align_u32()

Fixes warning that "negation of an unsigned value results in an
unsigned value".

Reviewed-by: Neil Roberts <nroberts@igalia.com>

src/compiler/spirv/vtn_private.h

@@ -732,7 +732,7 @@ void vtn_handle_decoration(struct vtn_builder *b, SpvOp opcode,
 static inline uint32_t
 vtn_align_u32(uint32_t v, uint32_t a)
 {
-   assert(a != 0 && a == (a & -a));
+   assert(a != 0 && a == (a & -((int32_t) a)));
    return (v + a - 1) & ~(a - 1);
 }