chinese/pyzy: Fix build on powerpc64, aarch64 and armv*

Add explicit signed to char to fix build where char is unsigned.
./DoublePinyinTable.h:166:5: error: constant expression evaluates to -1 which cannot be narrowed to type 'char' [-Wc++11-narrowing]

PR: 240457
Approved by: linimon (mentor), henry.hu.sh@gmail.com (maintainer)