nettle: fix missing link to libm

nettle-benchmark uses ldexp without explicitly linking to libm, fixes:
http://autobuild.buildroot.net/results/c8e64fabcb98b1ff4bf737e2181b7cf2ae731550

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/nettle/nettle-Added-lm-when-linking-nettle-benchmark.patch

@@ -0,0 +1,17 @@
+Added -lm when linking nettle-benchmark.
+
+Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
+
+diff --git a/examples/Makefile.in b/examples/Makefile.in
+index 909fdda..3becd08 100644
+--- a/examples/Makefile.in
++++ b/examples/Makefile.in
+@@ -11,7 +11,7 @@ PRE_CPPFLAGS = -I.. -I$(top_srcdir)
+ PRE_LDFLAGS = -L..
+ 
+ OPENSSL_LIBFLAGS = @OPENSSL_LIBFLAGS@
+-BENCH_LIBS = @BENCH_LIBS@
++BENCH_LIBS = @BENCH_LIBS@ -lm
+ 
+ RSA_TARGETS = rsa-keygen$(EXEEXT) rsa-sign$(EXEEXT) \
+ 	      rsa-verify$(EXEEXT) rsa-encrypt$(EXEEXT) rsa-decrypt$(EXEEXT)