RFR(T): 8221434: Fix typo in lib-x11 autoconf error message about missing headers

[Gustavo Romero]

Hi,

Could the following trivial change be reviewed, please?

bug    : https://bugs.openjdk.java.net/browse/JDK-8221434
webrev : http://cr.openjdk.java.net/~gromero/8221434/v1/

Thank you and best regards,
Gustavo