util: Remove u_math from u_vector

u_vector.h doesn't actually use anything from u_math, but it does mean
everyone has to pull in src/gallium/auxiliary/util includes.

Just remove it, adding a <string.h> include to u_vector.c to cover
memcpy.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/util/u_vector.h

@@ -31,7 +31,6 @@
 
 #include <stdint.h>
 #include <stdlib.h>
-#include "util/u_math.h"
 #include "util/macros.h"
 
 /* TODO - move to u_math.h - name it better etc */