build-sys: set -e separately, rather than putting -e in the shebang line

Like a008d76 (autogen.sh: set -e separately, rather than putting -e in
the shebang line) in libabc by Josh Triplett.
diff --git a/bootstrap b/bootstrap
index e6d8260..d50dae9 100755
--- a/bootstrap
+++ b/bootstrap
@@ -1,4 +1,5 @@
-#!/bin/sh -e
+#!/bin/sh
+set -e
 
 gtkdocize --docdir libkmod/docs || touch libkmod/docs/gtk-doc.make
 autoreconf --install --symlink