[llvm] [cmake] Include httplib in LLVMConfig.cmake (#79305)

Include LLVM_ENABLE_HTTPLIB along with httplib package finding in
LLVMConfig.cmake, as this dependency is needed by LLVMDebuginfod that is
now used by LLDB. Without it, building LLDB standalone fails with:

```
CMake Error at /usr/lib/llvm/19/lib64/cmake/llvm/LLVMExports.cmake:90 (set_target_properties):
  The link interface of target "LLVMDebuginfod" contains:

    httplib::httplib

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  /usr/lib/llvm/19/lib64/cmake/llvm/LLVMConfig.cmake:357 (include)
  cmake/modules/LLDBStandalone.cmake:9 (find_package)
  CMakeLists.txt:34 (include)
```