use -Werror=implicit-function-declaration for function detection

the previous -Werror triggers too easily: on NixOS for example the
FORTIFY_SOURCE #warning about a missing optimization level breaks all
the checks when using -O0 (which is the default for non-release builds).

configure

@@ -146,7 +146,7 @@ NEED_OPENBSD_SOURCE=0
 NEED_LIBBSD_OPENBSD_VIS=0
 
 COMPATS=
-COMP="${CC} ${CFLAGS} -Wno-unused -Werror"
+COMP="${CC} ${CFLAGS} -Werror=implicit-function-declaration"
 
 # singletest name var extra-cflags extra-libs msg
 singletest() {

have/landlock.c

@@ -19,6 +19,8 @@
 #include <sys/stat.h>
 #include <sys/syscall.h>
 
+#include <stddef.h>
+
 #ifndef landlock_create_ruleset
 static inline int
 landlock_create_ruleset(const struct landlock_ruleset_attr *attr, size_t size,