Fixed typo in one of the ifndef's in pool.h used to enable/disable debug mode
diff --git a/src/pool.h b/src/pool.h
index 1cae48f..e0fafa9 100644
--- a/src/pool.h
+++ b/src/pool.h
@@ -124,7 +124,7 @@ extern char *git_pool_strcat(git_pool *pool, const char *a, const char *b);
/*
* Misc utilities
*/
-#ifndef _DEBUG_POOL
+#ifndef GIT_DEBUG_POOL
extern uint32_t git_pool__open_pages(git_pool *pool);
#endif
extern bool git_pool__ptr_in_pool(git_pool *pool, void *ptr);