git_sha1_gen: fix output on python3

String handling has changed on python3.

Before this patch, on python3:
	#define MESA_GIT_SHA1 "git-b'b99dcbfeb3'"
After:
	#define MESA_GIT_SHA1 "git-b99dcbfeb3"

(No change on python2, it always looked ok)

Cc: Jose Fonseca <jfonseca@vmware.com>
Fixes: b99dcbfeb3 "build: Convert git_sha1_gen script to Python."
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

bin/git_sha1_gen.py

@@ -12,7 +12,7 @@ try:
         'rev-parse',
         '--short=10',
         'HEAD',
-    ], stderr=open(os.devnull, 'w'))
+    ], stderr=open(os.devnull, 'w')).decode("ascii")
 except:
     # don't print anything if it fails
     pass