xburst: fix kernel build

Changeset r43049 removed the "062-mips_decompressor_build_fix.patch" which is
still required to properly build the xburst kernel.

Fix the syntax error in the preprocessed code by explicitely undefining the
"memcpy" and "memset" macros prior to declaring the replacements functions.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44133