classes/populate_sdk_ext: fix cascading from preparation failure

classes/populate_sdk_ext: fix cascading from preparation failure

During extensible SDK installtion, if the build system preparation step
fails we try to put something at the end of the environment setup script
to show an error when it is sourced, in case the user doesn't realise
that the partially-installed SDK is broken. However, an apostrophe in
the message (actually a single quote) appears to terminate the string
and therefore breaks the command. Drop it to avoid that.

(From OE-Core rev: 21e591d182e24c399ae010a8eff9b89947061a46)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>