Change index to strchr for clang

[john-peterson]

https://stackoverflow.com/questions/4091864/c-differences-between-strchr-and-index

[john-peterson]

make
dsub.c:98:31: error: call to undeclared library function 'index' with type 'char *(const char *, int)'; ISO C99 and later do not suppo...

cc -v
clang version 16.0.6
Target: aarch64-unknown-linux-android24
Thread model: posix
InstalledDir: /data/data/com.termux/files/usr/bin