Add .gitignore file

Ignored are all files created when building from a git checkout as well as
anything deleted by 'make clean'.

Signed-off-by: Ben Gardner <gardner.ben@gmail.com>

.gitignore

+*~
+*.o
+*.a
+*.da
+*.bb
+*.bbg
+*.prof
+/autom4te.cache
+/config.log
+/config.status
+/dbclient
+/dropbear
+/dropbearconvert
+/dropbearkey
+/dropbearmulti
+/scp
+/scp-progress
+Makefile
+config.h
+config.h.in
+configure