Report Abseil as a required CMake dependency

The configuration summary listed Abseil among the optional packages when
it was provided by the bundled submodule, since no REQUIRED package
lookup was performed in that case. Ceres cannot be built without Abseil,
so declare the package as required regardless of how it was obtained.

Change-Id: I8b61d3c53fefe79412dc5d0ca7a5e5d4cb211e40

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f80f9cc..2e81b77 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -827,6 +827,7 @@
 
 set_package_properties(absl PROPERTIES
   DESCRIPTION "${CERES_ABSL_DESCRIPTION}"
+  TYPE REQUIRED
   URL "https://abseil.io")
 set_package_properties(AccelerateSparse PROPERTIES
   DESCRIPTION "Apple's Accelerate framework with sparse linear solvers")