samples/posix: eventfd: Prevent run in twister when SRAM below 32K

This test makes use of dynamic RAM allocation done on the leftover
SRAM. Similarly to test/posix/eventfd, limit this test execution on
platforms with at least 32K of RAM available.

Fixes #38601

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>