Added NXT_MAYBE_UNUSED for __attribute__((__unused__)).

When testing some configurations of compilers and OSes, I noticed
that clang(1) 13 on Debian caused a function to be compiled but
unused, and the compiler triggered a compile error.

To avoid that error, use __attribute__((__unused__)).  Let's call
our wrapper NXT_MAYBE_UNUSED, since it describes itself more
precisely than the GCC attribute name.  It's also the name that
C2x (likely C23) has given to the standard attribute, which is
[[maybe_unused]], so it's also likely to be more readable because
of that name being in ISO C.

src/nxt_conf_validation.c

@@ -77,7 +77,8 @@ static nxt_int_t nxt_conf_vldt_error(nxt_conf_validation_t *vldt,
 static nxt_int_t nxt_conf_vldt_var(nxt_conf_validation_t *vldt, nxt_str_t *name,
     nxt_str_t *value);
 nxt_inline nxt_int_t nxt_conf_vldt_unsupported(nxt_conf_validation_t *vldt,
-    nxt_conf_value_t *value, void *data);
+    nxt_conf_value_t *value, void *data)
+    NXT_MAYBE_UNUSED;
 
 static nxt_int_t nxt_conf_vldt_mtypes(nxt_conf_validation_t *vldt,
     nxt_conf_value_t *value, void *data);