samples: basic: hash_map: fix libc heap size setting

The malloc arena/heap size setting can be adjusted using different
Kconfig options, depending on the libc implementation. This means
prj.conf can't be used to set this value on projects that can be built
for multiple libcs without generating a Kconfig warning.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>