nettle: enable fat build

This allows to include optimizations such as ARM neon which
are detected on run-time.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
[Jo-Philipp Wich: picked from openwrt#191 and rebased onto LEDE master]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/libs/nettle/Makefile

@@ -40,6 +40,7 @@ TARGET_CFLAGS += $(FPIC)
 
 CONFIGURE_ARGS += \
 	--enable-shared \
+	--enable-fat \
 	--disable-openssl \
 	--disable-documentation \
 	--enable-static