[CMake][Fuchsia] Add LLVM_ENABLE_HTTPLIB to Stage 2 build

This patch sets the LLVM_ENABLE_HTTPLIB flag to ON in the stage 2 build
similar to how many of the other dependency flags are already specified.
This is necessary to configure the stage 2 build by itself, otherwise
the CMake configuration crashes.

This is currently causing the MLGO demo to fail since we're only using
stage 2 to avoid having to build stage 1 to save some compile time.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D152057