Ignore *.o.ur-safe build artifacts

Generated when building on Ubuntu 18.04.  Also ignore the new
dynamically generated zfs-mount-generator.8 man page, and the
module/.cache.mk file.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7534

module/.gitignore

@@ -6,6 +6,7 @@
 .*.cmd
 .*.d
 
+/.cache.mk
 /.tmp_versions
 /Module.markers
 /Module.symvers