Makefile.am: Fix "permission denied" during "make distcheck"

This fixes the following error during "make distcheck":
	echo 0.5.3.24-2a36 > ../../.version-t && mv ../../.version-t ../../.version
	/bin/sh: ../../.version-t: Permission denied

Makefile.am

@@ -19,7 +19,7 @@ install-udev-rules:
 uninstall-udev-rules:
 	rm -rf /etc/udev/rules.d/rtl-sdr.rules
 
-EXTRA_DIST = git-version-gen
+EXTRA_DIST = git-version-gen .version
 
 if HAVE_DOXYGEN