cmFunctionBlocker: include missing header

This is needed to have `std::string::operator==` used in the code. Mark
it as such since older IWYU releases don't recognize this.

Source/cmFunctionBlocker.cxx

@@ -5,6 +5,7 @@
 #include <cassert>
 #include <memory> // IWYU pragma: keep
 #include <sstream>
+#include <string> // IWYU pragma: keep
 #include <utility>
 
 #include "cmExecutionStatus.h"