Use local paths for including headers in examples and tests

This makes sure a plain "make" succeeds without doing "make install".

examples/hogweed-benchmark.c

@@ -34,12 +34,12 @@
 
 #include <time.h>
 
-#include <nettle/dsa.h>
-#include <nettle/rsa.h>
+#include "dsa.h"
+#include "rsa.h"
 
-#include <nettle/nettle-meta.h>
-#include <nettle/sexp.h>
-#include <nettle/knuth-lfib.h>
+#include "nettle-meta.h"
+#include "sexp.h"
+#include "knuth-lfib.h"
 
 #include "../ecdsa.h"
 #include "../ecc-internal.h"

testsuite/ecdsa-keygen-test.c

 #include "testutils.h"
-#include <nettle/knuth-lfib.h>
+#include "knuth-lfib.h"
 
 /* Check if y^2 = x^3 - 3x + b */
 static int