[Support] [Debuginfod] Use libcurl imported library.

A [[ https://lists.llvm.org/pipermail/llvm-dev/2021-December/154203.html | report on llvm-dev ]] indicated that `curl.h` could be found during build configuration but not during compilation. This diff uses the libcurl imported library created by findcurl, so that includes and libs are automatically managed correctly by cmake.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D115189