Add nagios-plugins.spec file to .gitignore

The nagios-plugins.spec file is created from nagios-plugins.spec.in and
should be ignored by Git.

.gitignore

@@ -20,6 +20,7 @@ NP-VERSION-FILE
 /Makefile
 /Makefile.in
 /missing
+/nagios-plugins.spec
 /stamp-h1
 /test.pl
 /THANKS