Made CMakeLists.txt compatible with older versions of CMake.

By replacing DIRECTORY by PATH:
http://www.cmake.org/cmake/help/v3.0/command/get_filename_component.html

Change-Id: I6f5802deeda53dea443b255294235e43e7bb5389
Tested: on Linux.

CMakeLists.txt

@@ -72,7 +72,7 @@ include_directories(include)
 add_executable(flatc ${FlatBuffers_Compiler_SRCS})
 
 function(compile_flatbuffers_schema_to_cpp SRC_FBS)
-  get_filename_component(SRC_FBS_DIR ${SRC_FBS} DIRECTORY)
+  get_filename_component(SRC_FBS_DIR ${SRC_FBS} PATH)
   string(REGEX REPLACE "\\.fbs$" "_generated.h" GEN_HEADER ${SRC_FBS})
   add_custom_command(
     OUTPUT ${GEN_HEADER}