fix undefined reference to dlopen, etc

[junrushao]

Occasionally seeing complaints in the discussion forum about the following error:

libtvm.so: undefined reference to `dlopen'
libtvm.so: undefined reference to `dlclose'
libtvm.so: undefined reference to `dlerror'
libtvm.so: undefined reference to `dlsym'

This is due to missing -ldl when using compilers like clang. The more cmake way to address this should be adding the ${CMAKE_DL_LIBS} flag directly.

CC: @icemelon9

[junrushao]

@icemelon9 Hey could you review?

[icemelon]

Thanks @junrushao1994