flowgrindd: fix compiler warning

This patch should fix the following compiler warning, when compiled with debug on 64bit:

src/daemon.c: In function ‘try_read_n_bytes’:
src/daemon.c:1070:3: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 10 has type ‘size_t’ [-Wformat=]
   DEBUG_MSG(LOG_NOTICE, "flow %d received cmsg: type = %u, "
   ^