package/poco: Update check for uclibc toolchain

LIBC is really only used to construct the target tuple. To check if we
are using a uClibc toolchain, we use BR2_TOOLCHAIN_USES_UCLIBC.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>