libcgroup: Use GNU strerror_r only when its available

This patch ensures that a fallback to use XSI compliant strerror_r when
GNU extention is not implemented, which fixes the case when building
with musl

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>