Config.mk: add XEN_LOG_DIR to BUILD_MAKE_VARS

... so that it can be turned into shell environment variable and
exported to header files.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Config.mk

@@ -181,7 +181,7 @@ endef
 
 BUILD_MAKE_VARS := sbindir bindir LIBEXEC LIBEXEC_BIN libdir SHAREDIR \
                    XENFIRMWAREDIR XEN_CONFIG_DIR XEN_SCRIPT_DIR XEN_LOCK_DIR \
-                   XEN_RUN_DIR XEN_PAGING_DIR XEN_DUMP_DIR
+                   XEN_RUN_DIR XEN_PAGING_DIR XEN_DUMP_DIR XEN_LOG_DIR
 
 buildmakevars2file = $(eval $(call buildmakevars2file-closure,$(1)))
 define buildmakevars2file-closure