Makefile: make "git/config.mak.uname" inclusion optional

If we haven't got a "git" directory, it should still be possible to run
"make get-git", so we cannot include this file unconditionally.

Signed-off-by: John Keeping <john@keeping.me.uk>

Makefile

@@ -33,7 +33,7 @@ DOC_PDF  = $(patsubst %.txt,%.pdf,$(MAN_TXT))
 
 #-include config.mak
 
-include git/config.mak.uname
+-include git/config.mak.uname
 #
 # Let the user override the above settings.
 #