[makefile] Quote argument to sed containing []

[] are shell metacharacters, and they can get confused by files in the
local directory.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

src/Makefile

@@ -71,7 +71,7 @@ noargs : blib $(BIN)/NIC $(BIN)/gpxe.dsk $(BIN)/gpxe.iso $(BIN)/gpxe.usb $(BIN)/
 # If no architecture is specified in Config or on the command-line,
 # use that of the build machine.
 #
-ARCH		:= $(shell uname -m | sed -e s,i[3456789]86,i386,)
+ARCH		:= $(shell uname -m | sed -e 's,i[3456789]86,i386,')
 
 # handle x86_64 like i386, but set -m32 option for 32bit code only
 ifeq ($(ARCH),x86_64)