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)