kconfig: fix menuconfig

"depends on" cannot use expressions as arguments, this statement
didn't actually do anything for kconfiglib.py and caused an
error with menuconfig.

There's currently no way to depend on a system heap being
present, however forthcoming patches move this feature away
from using the system heap anyway.

For now, if the user enables dynamic objects without defining
a system heap, they will get a build error.

Fixes #7266.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>