libzrtp: add .gitignore files

libs/libzrtp/.gitignore

+# -*- mode:conf -*-
+/include/zrtp_config_unix.h

libs/libzrtp/projects/gnu/.gitignore

+# -*- mode:conf -*-
+/Makefile
+/Makefile.in
+/_configs.sed
+/build/Makefile
+/build/Makefile.in
+/build/test/Makefile
+/build/test/Makefile.in
+/config/config.h
+/config/stamp-h1

libs/libzrtp/third_party/bnlib/.gitignore

+# -*- mode:conf -*-
+/bnconfig.h