Tests: Fix RunCMake.GenerateExportHeader use of size_t

Since commit ddcd1469e8 (MSYS: Add support for running under MSYS
runtime environment, 2021-04-01) the test uses `std::size_t` without
including `<cstddef>`.  Also, the type does not exist under `std::`
on VS 6.  Use just `size_t` instead.

Tests/RunCMake/GenerateExportHeader/exportheader_test.cpp

@@ -10,7 +10,7 @@
 
 static void rtrim(std::string& str, char byte)
 {
-  const std::size_t size = str.size();
+  size_t const size = str.size();
   if (size && str[size - 1] == byte) {
     str.resize(size - 1);
   }