prbs-tool: Don't require C99
prbs-tool: Don't require C99

from a Debian8 build:

[  256s] prbs-tool.c: In function 'apply_errors_prbs':
[  256s] prbs-tool.c:190:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
[  256s]   for (int i = 0; i < sizeof(pchan->prbs_u)-4; i++) {
[  256s]   ^

Change-Id: I8541fce50405525efffb12210a269e0ab8a687ab

osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx