[CMake] Don't look for terminfo libs when LLVM_ENABLE_TERMINFO=OFF

D85820 introduced a bug where LLVM_ENABLE_TERMINFO was set to true when
the library was found, even when the user had set
-DLLVM_ENABLE_TERMINFO=OFF.

Patch By: haampie

Differential Revision: https://reviews.llvm.org/D86173