stream_salsa20: amd64: correctly define symbol types

The crypto_stream_salsa20{,_xor} functions are broken on amd64 if libsodium is used as a shared library, as the symbol types in the assembly implementation aren't defined. This causes different warnings (depending on the toolchain used) during linking and/or runtime when you build a program using them, and leads to a segfault when trying to call them.

This patch fixes the problem.