change local lib headers to be relative for those pointing at "include/" dir

added relative paths to header files imported from src directory

added include guards for contrib/ files when referring to secp256k1.h

src/bench_internal.c

@@ -6,8 +6,8 @@
 #include <stdio.h>
 
 #include "secp256k1.c"
+#include "../include/secp256k1.h"
 
-#include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"
 #include "hash_impl.h"