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/schnorrsig/tests_exhaustive_impl.h

@@ -8,7 +8,7 @@
 #define SECP256K1_MODULE_SCHNORRSIG_TESTS_EXHAUSTIVE_H
 
 #include "../../../include/secp256k1_schnorrsig.h"
-#include "src/modules/schnorrsig/main_impl.h"
+#include "main_impl.h"
 
 static const unsigned char invalid_pubkey_bytes[][32] = {
     /* 0 */