gitignore: compile_commands.json is only top-level

Change-Id: I313b86d8736e948d52d57bad77550f2969c9fc67
Reviewed-on: https://gerrit.libreoffice.org/39107Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Muhammet Kara <muhammet.kara@pardus.org.tr>

.gitignore

@@ -173,4 +173,4 @@ LibreOffice.VC.VC.opendb
 *.sublime-workspace
 
 # vim-ide-integration
-compile_commands.json
+/compile_commands.json