mmc: omap_hsmmc: fix dependency on indirect header inclusion

omap_hsmmc.c depends on <linux/sizes.h> being included indirectly by
another header. Once we enable COMPILE_TEST for this driver, we might
compile under architectures which won't include <linux/sizes.h> for us.
In fact, one such case is x86.

In order to prevent compile breakages, let's explicitly include
<linux/sizes.h>.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>