kvm_write: fix -Wcast-qual warning in pointer arithmetic argument

Cast buf to `const char *` when doing arithmetic operation to match
`cp`'s type [1].

Differential Revision:	D10082
MFC after:	1 week
Reviewed by:	cem (earlier diff), vangyzen
Submitted by:	cem [1]
Sponsored by:	Dell EMC Isilon

lib/libkvm/kvm.c

@@ -476,7 +476,7 @@ kvm_write(kvm_t *kd, u_long kva, const void *buf, size_t len)
 		len -= cw;
 	}
 
-	return (cp - (char *)buf);
+	return (cp - (const char *)buf);
 }
 
 int