util/cbfstool/lz4frame.c: Add comment to fall through

GCC7 has a new feature called -Wimplicit-fallthrough enabled by
default which checks for fallthrough in switch statements which is a
common error. When a fallthrough is actually intended a comment saying
so will satisfy GCC.

Fixes cbfstool not building with GCC7.

Change-Id: I83252fc96be7ce0971d4251b0fc88fbbd7440e71
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/20036
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbfstool/lz4/lib/lz4frame.c

@@ -1091,7 +1091,7 @@ size_t LZ4F_decompress(LZ4F_decompressionContext_t decompressionContext,
                 dctxPtr->tmpInTarget = minFHSize;   /* minimum to attempt decode */
                 dctxPtr->dStage = dstage_storeHeader;
             }
-
+	    /* Falls through. */
         case dstage_storeHeader:
             {
                 size_t sizeToCopy = dctxPtr->tmpInTarget - dctxPtr->tmpInSize;