Makefile: disable testsuite and example

We do not need them in the context of Buildroot.

Also, they break for a shared-only build (but it's fixed in patch 0001).

Signed-off-by:  "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN nettle-2.7.1.orig/Makefile.in nettle-2.7.1/Makefile.in
--- nettle-2.7.1.orig/Makefile.in	2013-05-28 16:21:54.000000000 +0200
+++ nettle-2.7.1/Makefile.in	2014-12-12 19:57:12.499805574 +0100
@@ -19,7 +19,7 @@
 
 FAT_TEST_LIST = @FAT_TEST_LIST@
 
-SUBDIRS = tools testsuite examples
+SUBDIRS = tools
 
 include config.make