This allows the compiler to inline the siphash24*() functions
for nm_hash_ptr() and nm_hash_str() (even without LTO).

This of course only applies to nm_hash_ptr() and nm_hash_str(),
which are implemented in "nm-hash-utils.c" itself. All other
nm_hash_*() functions are inline functions in "nm-hash-utils.h",
and thus these functions can be inlined instead. That is, in
other cases, the nm_hash_*() function instead can be inlined.
For nm_hash_ptr() and nm_hash_str() instead we want to inline the
siphash24*() functions.

So, no longer compile "siphash24.c" directly. Instead, only
build "nm-hash-utils.c" which internally #include "siphash24.c".