Set CMP0135 to improve FetchContent cache

Without it, make fails to redownload flatbuffers when changing between
branches during development. It can only be set if the version of cmake
is sufficiently high.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I686d9995ca0b387867a48d871a2f6eca62422952