openssl: Fix x86_64 build on some 64bit host systems

On some build hosts openssl fails to install since openssl installs itself into
lib64 while the openwrt Makefile expects the libs to end up in lib.

install -m0644 .../openwrt/build_dir/target-x86_64_uClibc-0.9.33.2/openssl-1.0.1e/ipkg-install/usr/lib/libcrypto.so.* .../openwrt/build_dir/target-x86_64_uClibc-0.9.33.2/openssl-1.0.1e/ipkg-x86_64/libopenssl/usr/lib/
install: cannot stat '.../openwrt/build_dir/target-x86_64_uClibc-0.9.33.2/openssl-1.0.1e/ipkg-install/usr/lib/libcrypto.so.*': No such file or directory
make[2]: *** [/openwrt/bin/x86_64/packages/libopenssl_1.0.1e-2_x86_64.ipk] Error 1
make[2]: Leaving directory `/openwrt/package/libs/openssl'
make[1]: *** [package/libs/openssl/compile] Error 2
make[1]: Leaving directory `/openwrt'

Set LIBDIR accordingly to fix this.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39885 3c298f89-4303-0410-b956-a3cf2f4a3e73