cmTargetExport: Analyze with IWYU

Since there is no cmTargetExport.cxx, cmTargetExport.h was not
being analyzed by IWYU. Associate cmTargetExport.h with
cmExportSet.cxx.

Source/cmExportSet.cxx

@@ -1,6 +1,6 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
-#include "cmExportSet.h"
+#include "cmExportSet.h" // IWYU pragma: associated
 
 #include <algorithm>
 #include <tuple>
@@ -11,7 +11,7 @@
 #include "cmMessageType.h"
 #include "cmStringAlgorithms.h"
 #include "cmTarget.h"
-#include "cmTargetExport.h"
+#include "cmTargetExport.h" // IWYU pragma: associated
 
 cmExportSet::cmExportSet(std::string name)
   : Name(std::move(name))

Source/cmTargetExport.h

@@ -4,6 +4,7 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
+#include <map>
 #include <string>
 
 class cmFileSet;