package/libxmlrpc: fix check-package warnings

Assignment in conditional blocks must use append-assignment.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 92d73f31b2ffdbbdb7049a9c25669c973766d7f9)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libxmlrpc/libxmlrpc.mk

@@ -40,10 +40,10 @@ LIBXMLRPC_STATIC_OPTS = SHARED_LIB_TYPE=NONE MUST_BUILD_SHLIB=N
 endif
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
-LIBXMLRPC_DEPENDENCIES = host-pkgconf openssl
-LIBXMLRPC_CONF_OPTS = --enable-abyss-openssl
+LIBXMLRPC_DEPENDENCIES += host-pkgconf openssl
+LIBXMLRPC_CONF_OPTS += --enable-abyss-openssl
 else
-LIBXMLRPC_CONF_OPTS = --disable-abyss-openssl
+LIBXMLRPC_CONF_OPTS += --disable-abyss-openssl
 endif
 
 LIBXMLRPC_MAKE_OPTS += $(LIBXMLRPC_STATIC_OPTS)