TestBigEndian: Include CheckTypeSize outside macro

When using https://github.com/cristianadam/cmake-checks-cache I have
noticed that CheckTypeSize would in certain cases have an empty
`__check_type_size_dir` variable.  The errors would point to
`TestBigEndian`.  By moving `include(CheckTypeSize)` outside the macro,
the errors go away.

Including dependencies of a module when the module is first included is
simpler and cleaner anyway.

Modules/TestBigEndian.cmake

@@ -14,6 +14,8 @@
 #   TEST_BIG_ENDIAN(VARIABLE)
 #   VARIABLE - variable to store the result to
 
+include(CheckTypeSize)
+
 macro(TEST_BIG_ENDIAN VARIABLE)
   if(NOT DEFINED HAVE_${VARIABLE})
     message(STATUS "Check if the system is big endian")
@@ -27,8 +29,6 @@ macro(TEST_BIG_ENDIAN VARIABLE)
       message(FATAL_ERROR "TEST_BIG_ENDIAN needs either C or CXX language enabled")
     endif()
 
-    include(CheckTypeSize)
-
     CHECK_TYPE_SIZE("unsigned short" CMAKE_SIZEOF_UNSIGNED_SHORT LANGUAGE ${_test_language})
     if(CMAKE_SIZEOF_UNSIGNED_SHORT EQUAL 2)
       message(STATUS "Using unsigned short")