rsa: don't use malloc.h in rsa-sign.c

stdlib.h is the header for malloc since at least c89/c90.
There is no system malloc.h on OpenBSD and trying to use malloc.h here
falls back to the U-Boot malloc.h breaking the build of qemu_arm64.

Fixes: 336d4615 ("dm: core: Create a new header file for 'compat' features")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>