Add opam local switch in gitignore (xapi-project#6364)

When using an opam local switch, ignoring the _opam directory prevents
git from searching within it, reducing noise in search results.

Author: psafont

.gitignore

@@ -6,6 +6,7 @@ _coverage/
 *.install
 *.swp
 compile_flags.txt
+_opam
 
 # tests
 xapi-db.xml