Add missing error string in ABT_error_get_str

The error string for ABT_ERR_INV_MUTEX_ATTR was missing in  the lookup
table for error strings.

This lead to
 * segmentation fault when ABT_error_str is called with err=ABT_ERR_FEATURE_NA
 * incorrect error strings for err>ABT_ERR_INV_MUTEX

src/error.c

@@ -49,6 +49,7 @@ int ABT_error_get_str(int err, char *str, size_t *len)
         "ABT_ERR_INV_TASK",
         "ABT_ERR_INV_KEY",
         "ABT_ERR_INV_MUTEX",
+        "ABT_ERR_INV_MUTEX_ATTR",
         "ABT_ERR_INV_COND",
         "ABT_ERR_INV_RWLOCK",
         "ABT_ERR_INV_EVENTUAL",