Don't gitignore cmd/mtypes directory, just the binaries

The cmd/mtypes and cmd/avalanche directories were being inadvertently
gitignore'd because the .gitignore didn't specify that only these names
at the root of the project should be ignored.

Signed-off-by: Craig Ringer <[email protected]>

Author: ringerc

Diff for: .gitignore

@@ -1,5 +1,7 @@
 ./avalanche
 .build/
 .idea/
-mtypes
-avalanche
+/mtypes
+/avalanche
+/cmd/mtypes/mtypes
+/cmd/avalanche/avalanche