PrintModifierMapping: stop leaking the map returned by XGetKeyboardMapping

Resolves issue reported by Oracle Parfait static analyzer:

Error: Memory leak
   Memory leak [memory-leak] (CWE 401):
      Memory leak of pointer pointer allocated with XGetKeyboardMapping(...)
        at line 251 of app/xmodmap/exec.c in function 'PrintModifierMapping'.
          pointer allocated at line 222 with XGetKeyboardMapping(...)

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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