check-host-sed.sh: add missing shebang line

Bash seems to be smart enough to source the file when execve returns
ENOEXEC, but other shells might not be.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/dependencies/check-host-sed.sh

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 SEDLIST="/usr/bin/sed /bin/sed sed gnused gsed"
 
 for SED in $SEDLIST