Bump to lz4/lz4@d4437184 [v1.9.3] (#5420)

Bump from lz4 v1.9.2 to v1.9.3.

Changes: lz4/lz4@fdf2ef5...d443718

	% git diff --shortstat fdf2ef58...d4437184
	 94 files changed, 2917 insertions(+), 1377 deletions(-)

Note: lz4/lz4 has no `master` branch.  Release tags are for commits
on the `release` branch.  Update `.gitmodules` accordingly.

Author: jonpryor

.gitmodules

@@ -17,7 +17,7 @@
 [submodule "external/lz4"]
     path = external/lz4
     url = https://github.com/lz4/lz4.git
-    branch = master
+    branch = release
 [submodule "external/mman-win32"]
     path = external/mman-win32
     url = https://github.com/witwall/mman-win32.git