libtorrent: zlib is mandatory

zlib isn't optional even though it seems like it, fixes build failure
such as:

log.cc:51:18: fatal error: zlib.h: No such file or directory

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libtorrent/Config.in

@@ -3,6 +3,7 @@ config BR2_PACKAGE_LIBTORRENT
 	depends on BR2_INSTALL_LIBSTDCPP
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on BR2_ARCH_HAS_ATOMICS
+	select BR2_PACKAGE_ZLIB
 	help
 	  BitTorrent library written in C++ for *nix
 

package/libtorrent/libtorrent.mk

@@ -6,13 +6,12 @@
 
 LIBTORRENT_VERSION = 0.13.6
 LIBTORRENT_SITE = http://rtorrent.net/downloads
-LIBTORRENT_DEPENDENCIES = host-pkgconf \
-	$(if $(BR2_PACKAGE_OPENSSL),openssl) \
-	$(if $(BR2_PACKAGE_ZLIB),zlib)
+LIBTORRENT_DEPENDENCIES = host-pkgconf zlib \
+	$(if $(BR2_PACKAGE_OPENSSL),openssl)
 LIBTORRENT_CONF_OPTS = --enable-aligned \
 	--disable-instrumentation \
-	$(if $(BR2_PACKAGE_OPENSSL),--enable-openssl,--disable-openssl) \
-	$(if $(BR2_PACKAGE_ZLIB),--with-zlib=$(STAGING_DIR)/usr,--without-zlib)
+	--with-zlib=$(STAGING_DIR)/usr \
+	$(if $(BR2_PACKAGE_OPENSSL),--enable-openssl,--disable-openssl)
 LIBTORRENT_INSTALL_STAGING = YES
 LIBTORRENT_AUTORECONF = YES
 LIBTORRENT_LICENSE = GPLv2