RFR(XS): 8215009: GCC 8 compilation eror in libjli

[Dmitry Chuyko]

Hello,

Please review a small fix in java_md_solinux.c: continuation is not 
truly compatible with pthread_create start_routine's signature but we 
control what actually happens. So it makes sense to add intermediate 
void* cast to silence the error.

bug: https://bugs.openjdk.java.net/browse/JDK-8215009
webrev: http://cr.openjdk.java.net/~dchuyko/8215009/webrev.00/
testing: submit repo (mach5-one-dchuyko-JDK-8215009-20181207-1625-13615: 
PASSED)

-Dmitry