cmake: Fix static_assert in pw_iot_sdk_config

pw_iot_sdk_config/internal/validation.h depends on static_assert macro
which, for C, is provided by assert.h, and available when compiler mode
is at least C11.

Add an explicit dependency on the `c_static_assert` compile feature to
fix build errors when no other module enables it.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>