build: Fix #include "..." paths to get rid of further -I arguments

This simplifies building without a build system.

This is in line with #925; the paths fixed here were either forgotten
there or only introduced later. This commit also makes the Makefile
stricter so that further "wrong" #include paths will lead to build
errors even in autotools builds.

This belongs to #929.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/modules/ecdh/bench_impl.h

@@ -7,7 +7,7 @@
 #ifndef SECP256K1_MODULE_ECDH_BENCH_H
 #define SECP256K1_MODULE_ECDH_BENCH_H
 
-#include "../include/secp256k1_ecdh.h"
+#include "../../../include/secp256k1_ecdh.h"
 
 typedef struct {
     secp256k1_context *ctx;