Add external/lz4 to match internal manifest.

external/lz4 was added to the internal manifest but not the external one.

Bug: 26218766
TEST=repo sync; `make` on a board with squashfs enabled.

Change-Id: Iec9716b41794164f09f9a069425f367d1e4b77b1

default.xml

@@ -97,6 +97,7 @@
   <project path="external/libvpx" name="platform/external/libvpx" groups="bdk" />
   <project path="external/libweave" name="platform/external/libweave" groups="bdk" />
   <project path="external/llvm" name="platform/external/llvm" groups="bdk" />
+  <project path="external/lz4" name="platform/external/lz4" groups="bdk" />
   <project path="external/lzma" name="platform/external/lzma" groups="bdk" />
   <project path="external/lzop" name="platform/external/lzop" groups="bdk" />
   <project path="external/minijail" name="platform/external/minijail" groups="bdk" />