Fix declaration of seed48().

According to various sources, `seed48()` returns a pointer.

See:
http://www.thinkage.ca/english/gcos/expl/c/lib/seed48.html
https://developer.apple.com/library/ios/documentation/System/Conceptual/ManPages_iPhoneOS/man3/seed48.3.html
http://www-01.ibm.com/support/knowledgecenter/ssw_aix_71/com.ibm.aix.basetrf1/drand48.htm