tcg/mips: constify tcg_target_callee_save_regs

Reviewed-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

tcg/mips/tcg-target.inc.c

@@ -2341,7 +2341,7 @@ static const TCGTargetOpDef *tcg_target_op_def(TCGOpcode op)
     }
 }
 
-static int tcg_target_callee_save_regs[] = {
+static const int tcg_target_callee_save_regs[] = {
     TCG_REG_S0,       /* used for the global env (TCG_AREG0) */
     TCG_REG_S1,
     TCG_REG_S2,