samples: basic: hash_map increasing the stack size for cxx_unordered_map

When running the testcase libraries.hash_map.newlib.cxx_unordered_map.djb2
of this samples/basic/hash_map, it appears that some platforms
have a too low stack size.
This PR is increasing that value to pass the testcase.

Signed-off-by: Francois Ramu <francois.ramu@st.com>