rng-random: only build on POSIX platforms

There is no /dev/random on win32.

Cc: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

backends/Makefile.objs

-common-obj-y += rng.o rng-random.o rng-egd.o
+common-obj-y += rng.o rng-egd.o
+common-obj-$(CONFIG_POSIX) += rng-random.o