Improve coverage computation

tests/test-mbitset.c

@@ -21,8 +21,9 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 #include <stdio.h>
-
 #include "m-string.h"
+#include "coverage.h"
+
 #include "m-bitset.h"
 
 static void test1(void)