Fixed explicit instantiation problem when compiling as header-only.

When compiling tutorial 405_AsRigidAsPossible under linux with libigl as a header-only library, the default build configuration uses AVX for vectorization. The svd3x3_avx.cpp ended up being included (at least) twice and the double explicit instantiation fired an error in the compiler. Now, the explicit instantiations are guarded by #ifdef, so they only apply when compiling libigl as a static library.