vulkan/alloc: Use char * for pointer arithmetic

MSVC doesn't like arithmetic on void *.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9511>

src/vulkan/util/vk_alloc.h

@@ -199,7 +199,7 @@ vk_multialloc_alloc(struct vk_multialloc *ma,
                     const VkAllocationCallbacks *alloc,
                     VkSystemAllocationScope scope)
 {
-   void *ptr = vk_alloc(alloc, ma->size, ma->align, scope);
+   char *ptr = vk_alloc(alloc, ma->size, ma->align, scope);
    if (!ptr)
       return NULL;