Add Threads dependency in exported CMake config file

BoringSSL depends on multi-thread library provided by the OS.
However, this dependency is not transitively passed to the project
that depends on BoringSSL automatically. We are seeing curl 8.21
build failures on ToT BoringSSL on Windows due to this limitation.
This patch address this issue by adding "Threads" as a dependency
in exported CMake config.

Change-Id: I4c6d66ce491eb58efcbba9db44bb86584e1e3205
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/99307
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

cmake/OpenSSLConfig.cmake

diff --git a/cmake/OpenSSLConfig.cmake b/cmake/OpenSSLConfig.cmake
index 86632ac..5adc97d 100644
--- a/cmake/OpenSSLConfig.cmake
+++ b/cmake/OpenSSLConfig.cmake
@@ -17,6 +17,9 @@
 # exporters/cmake/OpenSSLConfig.cmake.in in OpenSSL.
 
 include(${CMAKE_CURRENT_LIST_DIR}/OpenSSLTargets.cmake)
+include(CMakeFindDependencyMacro)
+
+find_dependency(Threads)
 
 # Recursively collect dependency locations for the imported targets.
 macro(_openssl_config_libraries libraries target)