gallium/util: add missing u_debug include

gallium/util: add missing u_debug include

Needed for assert.
Fixes build on BE archs with -Werror=implicit-function-declaration.

In file included from
../../../../../src/gallium/auxiliary/draw/draw_fs.c:30:0:
../../../../../src/gallium/auxiliary/util/u_math.h: In function
'util_memcpy_cpu_to_le32':
../../../../../src/gallium/auxiliary/util/u_math.h:810:4: error:
implicit declaration of function 'assert'
[-Werror=implicit-function-declaration]
    assert(n % 4 == 0);
        ^

Cc: "10.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>