mongoose: correct openssl define

NS_ENABLE_SSL was renamed to MG_ENABLE_SSL in v6.0, commit e1dd3f06fe33
(Rename Mongoose constants: NS_ -> MG_, NSF_ -> MG_F_), so use the new name
instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mongoose/mongoose.mk

@@ -14,7 +14,7 @@ MONGOOSE_CFLAGS = $(TARGET_CFLAGS)
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 MONGOOSE_DEPENDENCIES += openssl
-MONGOOSE_CFLAGS += -DNS_ENABLE_SSL
+MONGOOSE_CFLAGS += -DMG_ENABLE_SSL
 endif
 
 define MONGOOSE_BUILD_CMDS