Add .sh to list of valid library extenions.
Add .sh to list of valid library extenions.

As discussed with mistym, it's not only compiled code that goes in
/usr/lib, but any types of libraries. As such, .pm was added to the
valid extensions list a while back. Unfortunately, .sh was not added,
even though configuration includes and platform function library shell
files are often put in /usr/lib as shellscript libraries. This patch
adds .sh to the list of valid library extensions.

Closes Homebrew/homebrew#14993.

Signed-off-by: Adam Vandenberg <34c2b6407fd5a10249a15d699d40f9ed1782e98c@gmail.com>

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