MurmurHash3: actually inline functions

"__attribute__((always_inline))" does not replace "inline" and they
still need to be used together. This fixes "always_inline function
might not be inlinable [-Wattributes]" warning in gcc 4.7

Idea-from-patch-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>