fix: Enable ABSL_ENABLE_INSTALL.

This makes all Abseil targets installable, which is required to make
targets that depend on those in this project installable as well.

third_party/CMakeLists.txt

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
+set(ABSL_ENABLE_INSTALL ON)
 if(NOT ${ABSL_INCLUDED_WITH_PROTOBUF})
  set(ABSL_PROPAGATE_CXX_STD ON)
  add_subdirectory(abseil-cpp)