remove warnings in tests

test/stralloc_buffer.c

@@ -1,7 +1,7 @@
 #include "stralloc.h"
 #include "buffer.h"
 
-main() {
+int main() {
   static stralloc sa;
   /* static makes sure sa is initialized and empty;
    * use stralloc_init to initialize and stralloc_copys(&sa,"") to empty */
@@ -9,4 +9,5 @@ main() {
     buffer_putsa(buffer_1,&sa);
     buffer_putnlflush(buffer_1);
   }
+  return 0;
 }