getsockopt: avoid compiler warning

getsockopt: avoid compiler warning

getsockopt.c: In function 'rpl_getsockopt':
getsockopt.c:51:7: warning: passing argument 4 of 'getsockopt' from
incompatible pointer type
c:\dev\msys_setup\mingw\bin\../lib/gcc/mingw32/4.5.0/../../../../include/winsock2.h:543:32:
note: expected 'char *' but argument is of type 'int *'

A simple cast works around the mingw signature lameness.  If only
they had picked void* instead of char*, like POSIX says...

* lib/getsockopt.c (rpl_getsockopt): Add a cast for mingw.
Reported by Matthias Bolte.

Signed-off-by: Eric Blake <eblake@redhat.com>