toolchain-external: fix find_sysroot

Commit 23c0e97b (toolchain-external: anchor sysroot regex with /)
tried to make the find-sysroot work more consistently, especially for
toolchains where the C library is located in a sub-directory, like the
"Realtek mips toolchain".

After that patch, the '/' that was trailing in the returned path got
removed now. This in turn breaks the Codesourcery toolchain.

We fix that by appending the now-missing trailing '/'.

Fixes:
http://autobuild.buildroot.net/results/9284d571668148febce23d96a9c0a97a6b2b43dc



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: 陈小 刚 <shawn_chen@realsil.com.cn>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>