MSVC: Fix casting warning C4057

test_tls.c(103,5): warning C4057: 'function': 'const unsigned char *' differs in indirection to slightly different base types from 'char [8]'
test_tls.c(104,5): warning C4057: 'function': 'const unsigned char *' differs in indirection to slightly different base types from 'char [9]'

Signed-off-by: Vitaly Chikunov <vt@altlinux.org>