Fetch RE2 in CMakeLists.txt (#4)

tcmalloc_profile_marshaler_test and profile_builder_test link against
re2::re2. Without FetchContent for RE2, CMake builds fail when RE2 is
not installed on the system.
PiperOrigin-RevId: 984090161

CMakeLists.txt

@@ -62,6 +62,14 @@ FetchContent_Declare(
 )
 FetchContent_MakeAvailable(protobuf)
 
+FetchContent_Declare(
+  re2
+  GIT_REPOSITORY https://github.com/google/re2.git
+  GIT_TAG        main
+  FIND_PACKAGE_ARGS NAMES re2
+)
+FetchContent_MakeAvailable(re2)
+
 FetchContent_Declare(
   benchmark
   GIT_REPOSITORY https://github.com/google/benchmark.git