carl9170: Fix minimum CMake version

The CMAKE_CURRENT_LIST_DIR command was added in version 2.8.4.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

carl9170fw/extra/FindGPERF.cmake

@@ -71,6 +71,8 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+cmake_minimum_required(VERSION 2.8.4)
+
 FIND_PROGRAM(GPERF_EXECUTABLE gperf DOC "path to the gperf executable")
 MARK_AS_ADVANCED(GPERF_EXECUTABLE)