random: add missing include xen/cache.h

random: add missing include xen/cache.h

The commit f6c9698 " x86: allow reading MSR_IA32_TSC with XENPF_resource_op"
introduced a built regression on ARM platform.

random.c:8:28: error: expected \91=\92, \91,\92, \91;\92, \91asm\92 or \91__attribute__\92 before \91boot_random\92
 unsigned int __read_mostly boot_random;
                            ^
The define __read_mostly is defined in asm/cache.h which is included by
other headers on x86 but not on ARM. Include xen/cache.h to fix the
build.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>