grep: remove musl build workaround

grep: remove musl build workaround

Commit de903755d79e (grep: fix build failure for musl) added
--with-included-regex to configure options in musl builds. The commit log
mentions build failure because grep configure doesn't detect the missing libc
regex support. Unfortunately, the commit log adds no details on said build
failure, or any autobuilder reference. The autobuilder didn't record
any grep build failure for version 2.20 that was current at the time.

Build with musl succeeds nowadays, so remove the workaround.

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>