bootutil: Only include <os/malloc.h> if it going to be used
bootutil: Only include <os/malloc.h> if it going to be used

When no dynamic memory is used, there is no need for include "os/malloc.h",
and do so will force the user to create an empty "os/malloc.h" file.

Signed-off-by: Carlos Falgueras García <10cdbf3f7c77bbc8f77bbb1feea5c7f1fa166d6d@wslw.es>

ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot