libmd: re-apply masking of _Transform names

Visibility can get complicated when, e.g., ifuncs are involved. In
particular, SHA256/SHA512 on aarch64 use ifuncs for their _Transform
implementations, which then exposes global symbols of the same name that
break things trying to statically link both libcrypto and libmd.

Revert this part of the _Transform removal to fix the pkg-static build
on aarch64.

Fixes: 81de655acd2 ("libmd: stop exporting Transform() symbols")