mcproxy: fix up sourcefilter patch

mcproxy: fix up sourcefilter patch

this spewed out 2 compile warnings

In file included from build_dir/target-mipsel_24kc_musl/mcproxy-2017-08-24-93b5ace42268160ebbfff4c61818fb15fa2d9b99/mcproxy/src/utils/mc_socket.cpp:43:0:

build_dir/target-mipsel_24kc_musl/mcproxy-2017-08-24-93b5ace42268160ebbfff4c61818fb15fa2d9b99/mcproxy/src/utils/sourcefilter.cpp: In function 'int setsourcefilter(int, uint32_t, const sockaddr*, socklen_t, uint32_t, uint32_t, const sockaddr_storage*)':
build_dir/target-mipsel_24kc_musl/mcproxy-2017-08-24-93b5ace42268160ebbfff4c61818fb15fa2d9b99/mcproxy/src/utils/sourcefilter.cpp:157:3: warning: this 'else' clause does not guard... [-Wmisleading-indentation]
   else
   ^~~~
build_dir/target-mipsel_24kc_musl/mcproxy-2017-08-24-93b5ace42268160ebbfff4c61818fb15fa2d9b99/mcproxy/src/utils/sourcefilter.cpp:160:5: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'else'
     int save_errno = errno;
     ^~~

Signed-off-by: John Crispin <john@phrozen.org>