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

.gitignore

@@ -14,6 +14,7 @@
 # Normal rules
 #
 *.[oa]
+*.o.ur-safe
 *.lo
 *.la
 *.mod.c