Remove unnecessary import of stdlib.h from revwalk.h

OS headers are best imported from a more central location anyway.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/revwalk.h

@@ -3,7 +3,6 @@
 
 #include "git/common.h"
 #include "git/revwalk.h"
-#include <stdlib.h>
 
 struct git_revpool {
 	git_odb *db;