* build: remove hardcoded lib/ destination
cmake internally has a logic to know where is the best place to install
a given library. For example on amd64 it does not make sense to install
the library in /usr/lib, this change allow the library to the installed
in /usr/lib64
* build: mark src/randomx.h as a public header
this allow src/randomx.h to be installed in /usr/include/randomx.h
automagically during make install
