Avoid compiler error implicit declaration of function Fixes errors like this:
nwall.c: In function ‘main’: nwall.c:112:13: error: implicit declaration of function ‘free’ [-Werror=implicit-function-declaration] 112 | free(message.iov_base); | ^~~~ nwall.c:34:1: note: include ‘<stdlib.h>’ or provide a declaration of ‘free’
