scons: insert 'git-' into MESA_GIT_SHA1 string

To match makefile build.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index 3d5d37a..abcce41 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -480,7 +480,7 @@
             f.close()
         return
 
-    commit = '#define MESA_GIT_SHA1 "%s"\n' % commit[0:7]
+    commit = '#define MESA_GIT_SHA1 "git-%s"\n' % commit[0:7]
     tempfile = "git_sha1.h.tmp"
     f = open(tempfile, "w")
     f.write(commit)