Fix compilation on GCC (test algebra_curves_static_test), we have problems with constexpr test now.

[martun]

GCC handles constexpr in some different way. We need to change the code somehow to make it happy.

Here's an error when building algebra_curves_static_test:

/home/martun/nil/solana-consensus-proof/libs/algebra/test/curves_static.cpp: In member function ‘void curves_manual_static_tests::curve_operation_test_mnt6_g2::test_method()’:
/home/martun/nil/solana-consensus-proof/libs/algebra/test/curves_static.cpp:817:27: error: non-constant condition for static assertion
817 | static_assert(p1 * C1 == p_mul_C, "mul error");