cmake: Fix googletest deprecated warnings by using target_compile_options()

cmake: Fix googletest deprecated warnings by using target_compile_options()

Previously, we attempted to disable deprecated declarations warnings when
building gtest by adding `-Wno-deprecated-declarations` to the COMPILE_OPTIONS
property of the googletest directory. However, this approach failed to apply
the option when actually building gtest.

This change applies the compile option directly to the `gtest` target using
target_compile_options() instead. Verified by forcing the condition to TRUE
and confirming the option is included when building `gtest-all.cc` through
`cmake --build ~/dev/ceph/build --target gtest --verbose`.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>