print/libraqm: fix build with GCC-based architectures

Don't shadow global variable index:
cc1: warnings being treated as errors
raqm.c: In function '_raqm_u32_to_u8_index':
raqm.c:1602: warning: declaration of 'index' shadows a global declaration

Fails to build with base GCC.

PR: 237949
Approved by: kwm (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20613