fix include paths in all the -zkp modules

This is causing out-of-tree build failures in Elements.

src/modules/surjection/surjection_impl.h

@@ -10,10 +10,10 @@
 #include <assert.h>
 #include <string.h>
 
-#include "eckey.h"
-#include "group.h"
-#include "scalar.h"
-#include "hash.h"
+#include "../../eckey.h"
+#include "../../group.h"
+#include "../../scalar.h"
+#include "../../hash.h"
 
 SECP256K1_INLINE static void secp256k1_surjection_genmessage(unsigned char *msg32, const secp256k1_generator *ephemeral_input_tags, size_t n_input_tags, const secp256k1_generator *ephemeral_output_tag) {
     /* compute message */