MAINTAINERS: add random.h headers to RNG subsection

Before, patches to two RNG header files wouldn't be sent to the right
recipients automatically. So add the two header files -- one for
userspace and one for the kernel -- to the right part of the MAINTAINERS
file.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

MAINTAINERS

@@ -18744,6 +18744,8 @@ S:	Maintained
 T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
 F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
 F:	drivers/char/random.c
+F:	include/linux/random.h
+F:	include/uapi/linux/random.h
 F:	drivers/virt/vmgenid.c
 F:	include/vdso/getrandom.h
 F:	lib/vdso/getrandom.c