tools: Download the defined gas-pp version instead of master

This fixes extracting of the tarball when master has progressed
from the specified hash.
Signed-off-by: Martin Storsjö <martin@martin.st>

extras/tools/packages.mak

@@ -32,8 +32,8 @@ XZ_URL=http://tukaani.org/xz/xz-$(XZ_VERSION).tar.bz2
 OPENSSL_VERSION=1.0.0e
 OPENSSL_URL=http://www.openssl.org/source/openssl-$(OPENSSL_VERSION).tar.gz
 
-GAS_URL=https://github.com/libav/gas-preprocessor/tarball/master
 GAS_VERSION=c76b628
+GAS_URL=https://github.com/libav/gas-preprocessor/tarball/$(GAS_VERSION)
 
 RAGEL_VERSION=6.8
 RAGEL_URL=http://www.complang.org/ragel/ragel-$(RAGEL_VERSION).tar.gz