util: fix MSVC warning in u_align_u32()

To silence
C:\Users\Brian\projects\mesa\src\util/u_vector.h(41) : warning C4146: unary
minus operator applied to unsigned type, result still unsigned

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

src/util/u_vector.h

@@ -38,7 +38,7 @@
 static inline uint32_t
 u_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);
 }