common: Add a new lz4.h header file

Add a header file to house the lz4 compression function. Add a comment
while we are here, since it not even clear from the name what the function
actuall does.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/lib/lz4_wrapper.c b/lib/lz4_wrapper.c
index 1c68e67..1e1e8d5 100644
--- a/lib/lz4_wrapper.c
+++ b/lib/lz4_wrapper.c
@@ -6,6 +6,7 @@
 #include <common.h>
 #include <compiler.h>
 #include <image.h>
+#include <lz4.h>
 #include <linux/kernel.h>
 #include <linux/types.h>