package/gcc: fix check-package

Commit dcaf6e75 (package/gcc: pass -Wno-error to debug builds)
introduced non-ASCII characters in a comment, copy-pasted from a
terminal output.

check-package does not like non-ASCII characters, and whines about
them.

Replace the fancy quotes by standard ASCII ones.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>