Revert "flowgrindd: fix compiler warning"

This reverts commit 6f969b5245c7f93d90a8d0452a2cdd66bd53627c.
Because the change caused the following warning on FreeBSD:

src/daemon.c:1072:30: warning: format specifies type 'size_t' (aka 'unsigned long') but the argument has type 'socklen_t' (aka 'unsigned int') [-Wformat]
                flow->id, cmsg->cmsg_type, cmsg->cmsg_len);
                                           ^~~~~~~~~~~~~~
./src/debug.h:53:46: note: expanded from macro 'DEBUG_MSG'
                        (unsigned int)pthread_self()%USHRT_MAX, ##__VA_ARGS__);\
flowgrindd: properly fix compiler warning

Apparently cmsg_len in defined as size_t in some newer glibc versions,
so cast it to socklen_t in order to fix the compiler warning.