add .gitattributes file

Git tends to see .depex files as text, causing hideous patches being
generated (and breaking PatchCheck.py).

Add a .gitattributes file instructing git to treat them as binary.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..ab4583b
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1 @@
+*.depex binary