Add .gitattributes file

On Windows, cmake/xxd.py generates text files with \r\n eol.
Because the in-repo files were stored with \n, and git considered
them as binary files, they were always considered out of date.

An alternative fix could be to let xxd.py generate binary files with \n.
But a .gitattributes file should also avoid people accidentally
converting line endings.

.gitattributes

@@ -0,0 +1,12 @@
+*.c                 text
+*.cpp               text
+*.h                 text
+*.cmake             text
+*.py                text
+*.txt               text
+*.sh                text
+*.vcxproj           text eol=crlf
+*.sln               text eol=crlf
+*.filters           text eol=crlf
+*.appxmanifest      text eol=crlf
+*.pbxproj           text