These warnings are promoted to errors with --enable-developer

dnstap.c: In function ‘send_dt’:
dnstap.c:435:3: error: implicit declaration of function ‘free’ [-Werror=implicit-function-declaration]
   free(buf);
   ^
dnstap.c:435:3: error: incompatible implicit declaration of built-in function ‘free’ [-Werror]
dnstap.c:435:3: note: include ‘<stdlib.h>’ or provide a declaration of ‘free’
dnstap.c:442:3: error: incompatible implicit declaration of built-in function ‘free’ [-Werror]
   free(buf);
   ^
dnstap.c:442:3: note: include ‘<stdlib.h>’ or provide a declaration of ‘free’
cc1: all warnings being treated as errors