Problem: indentation should be 4 spaces, no tabs

Solution: fix it

src/xpub.cpp

@@ -174,7 +174,7 @@ int zmq::xpub_t::xsetsockopt (int option_, const void *optval_,
 
         if (optvallen_ > 0) {
             int rc = welcome_msg.init_size(optvallen_);
-			errno_assert(rc == 0);
+            errno_assert(rc == 0);
 
             unsigned char *data = (unsigned char*)welcome_msg.data();
             memcpy(data, optval_, optvallen_);