<AWT Dev> RFR: 8225487: giflib legal file is missing attribution for openbsd-reallocarray.c

[Phil Race]

bug : https://bugs.openjdk.java.net/browse/JDK-8225487
webrev :  http://cr.openjdk.java.net/~prr/8225487/

A 2016 update to giflib brought in a new source file openbsd-reallocarray.c
We need attribution for this in the legal markdown file.
This update also changes the file to use the markdown ``` ..... ``` 
convention
instead of <pre> .... </pre>

-phil.