Fix TCC compilation

Author: hirrolot

tests/derive.c

@@ -116,4 +116,6 @@ int main(void) {
     { ML99_EVAL(DATATYPE99_assertAttrIsPresent(v(FOO))); }
 
 #undef FOO
+
+    return 0;
 }

tests/tests.c

@@ -336,4 +336,6 @@ int main(void) {
 
     UnitT dummy = unit_v;
     (void)dummy;
+
+    return 0;
 }