make: Specify the commit hash length for the bin-tarball name

Different versions of git have different default lengths. This uses a fixed
length hash prefix.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

Makefile

@@ -1,6 +1,6 @@
 #! /usr/bin/make
 VERSION_NAME=I Accidentally The Smart Contract
-VERSION=$(shell git describe --always --dirty=-modded)
+VERSION=$(shell git describe --always --dirty=-modded --abbrev=7)
 DISTRO=$(shell lsb_release -is 2>/dev/null || echo unknown)-$(shell lsb_release -rs 2>/dev/null || echo unknown)
 PKGNAME = c-lightning