make gen-version.sh run even if . is not in PATH

Signed-off-by: Michael Krelin <hacker@klever.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -15,7 +15,7 @@ CGIT_VERSION = 0.5
 all: cgit
 
 VERSION:
-	gen-version.sh
+	sh gen-version.sh
 
 -include VERSION