.gitignore: Ignore .obj files (#805)

EDK II windows build produces .obj files in source tree
diff --git a/.gitignore b/.gitignore
index e3a69e6..1e269cb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 # C
 *.o
+*.obj
 bin/
 buildfiles/
 **/obj/