tests: posix: headers: Add min_ram to fix nucleo_f103rb compilation error

Compilation of this test on nucleo_f103rb fails because "region 'RAM'
overflowed by 136 bytes". Since this board has a ram size of 20k, we set
the limit for this test at the next usual size 32k.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>