.gitignore: add missing ignores

Ignore output files created by "make defconfig; make".
Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

applets/.gitignore

 /applet_tables
 /usage
+/usage_pod

docs/.gitignore

 /busybox.1
 /BusyBox.html
+/busybox.net
 /BusyBox.txt
 /busybox.pod

include/.gitignore

 /config
 
+/applets.h
 /applet_tables.h
 /autoconf.h
+/bbconfigopts_bz2.h
 /bbconfigopts.h
+/NUM_APPLETS.h
 /usage_compressed.h
+/usage.h