Fixed compile warning related to file identification feature.

Change-Id: Id33cf778caa818c7d3988edee82058e63bfecbf0
Tested: on Linux

CMakeLists.txt

@@ -57,7 +57,7 @@ elseif(CMAKE_COMPILER_IS_GNUCXX OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
 endif()
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror -Wextra")
 
 if(FLATBUFFERS_CODE_COVERAGE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -fprofile-arcs -ftest-coverage")

include/flatbuffers/flatbuffers.h

@@ -613,7 +613,7 @@ class FlatBufferBuilder {
     return CreateVectorOfStructs(v.data(), v.size());
   }
 
-  static const int kFileIdentifierLength = 4;
+  static const size_t kFileIdentifierLength = 4;
 
   // Finish serializing a buffer by writing the root offset.
   // If a file_identifier is given, the buffer will be prefix with a standard