Revert "Remove the wrong heapsort implementation"

This reverts commit aa04b6ae33.

include/ucore/heapsort.h

@@ -0,0 +1,21 @@
+#ifndef UC_HEAPSORT_H_
+#define UC_HEAPSORT_H_
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+///compare function. Needs only to return < 0 if 
+//the first element is less than the second
+typedef int (*uc_hs_cmp)(const void *, const void *);
+
+void
+uc_heapsort(void *base, size_t count, size_t width,
+     uc_hs_cmp cmp);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif