don't include os_compat.h

When there are standardized headers, use these instead of the bloated
os_compat.h.

src/tag_pool.c

@@ -19,6 +19,8 @@
 #include "tag_pool.h"
 #include "utils.h"
 
+#include <assert.h>
+
 pthread_mutex_t tag_pool_lock = PTHREAD_MUTEX_INITIALIZER;
 
 #define NUM_SLOTS 4096