package/libxslt: fix static build

Set --with-libxml-libs-prefix so libtool finds the correct dependencies for
libxml2.

Fixes:
http://autobuild.buildroot.net/results/3f8/3f8d876ed6cfe6e73a75fa8904ebdd3f6f0fe53c/

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libxslt/libxslt.mk

@@ -14,7 +14,8 @@ LIBXSLT_CONF_OPTS = \
 	--with-gnu-ld \
 	--without-debug \
 	--without-python \
-	--with-libxml-prefix=$(STAGING_DIR)/usr/
+	--with-libxml-prefix=$(STAGING_DIR)/usr/ \
+	--with-libxml-libs-prefix=$(STAGING_DIR)/usr/lib
 LIBXSLT_CONFIG_SCRIPTS = xslt-config
 LIBXSLT_DEPENDENCIES = libxml2