lz4: Use a private header for U-Boot

At present U-Boot has a header file called lz4.h for its own use. If the
host has its own lz4 header file installed (e.g. from the 'liblz4-dev'
package) then host builds will use that instead.

Move the U-Boot file into its own directory, as is done with various
other headers with the same problem.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/compression.c

@@ -9,11 +9,11 @@
 #include <gzip.h>
 #include <image.h>
 #include <log.h>
-#include <lz4.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <asm/io.h>
 
+#include <u-boot/lz4.h>
 #include <u-boot/zlib.h>
 #include <bzlib.h>