Makefile: re-include cgit.conf in cgit.mk

This avoids needed to export every variable that might be used in
cgit.mk from the top-level Makefile.

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

cgit.mk

@@ -5,6 +5,8 @@ include Makefile
 
 CGIT_PREFIX = ../
 
+-include $(CGIT_PREFIX)cgit.conf
+
 # The CGIT_* variables are inherited when this file is called from the
 # main Makefile - they are defined there.