src/libadacl.c: cast two fgetxattr() and fsetxattr() params to size_t.

The two functions fgetxattr() and fsetxattr() take an unsigned "size"
parameter as arguments. We are passing them signed integers that we
happen to know are nonnegative, since we have ruled out the one
possible negative value -- but the compiler doesn't know that. To
avoid a warning from clang, we now cast the parameters to the
(unsigned) size_t type.