tests/c_lib/common/src/test_sqrt: Fix when double is not 64 bits

The combination of
32312692
74c9e7af
broke this test, when __SIZEOF_DOUBLE__ != 8.
In that case the test will fail to build,
as the variables the code uses are left ifdef'ed out.

Let's fix it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>