fixes #31 - undefined behaviour in AVX2 version

[jansturm92]

restrict keyword caused undefined compiler output since referenced
objects are overlapping in memory.

e.g, for

dilithium/avx2/sign.c

Line 159 in 497a98b

polyveck_power2round(&t1, &t0, &t1);

a1 and a in power2round_avx() point to the same object t1.

[coveralls]

Coverage remained the same at 94.537% when pulling 826120c on jansturm92:master into 497a98b on pq-crystals:master.

[gregorseiler]

Thank you for the pull request. These restrict qualifiers are removed in the round 3 code now.