powerpc: Fix unused variable warning for epapr_has_idle

This warning can be seen in allyesconfig, and was introduced by commit
f9eb581c63b2acce827570e105205c0789360650 "powerpc: fix build of
epapr_paravirt on 64-bit book3s".
Signed-off-by: NScott Wood <scottwood@freescale.com>

arch/powerpc/kernel/epapr_paravirt.c

@@ -30,7 +30,7 @@ extern u32 epapr_ev_idle_start[];
 #endif
 
 bool epapr_paravirt_enabled;
-static bool epapr_has_idle;
+static bool __maybe_unused epapr_has_idle;
 
 static int __init early_init_dt_scan_epapr(unsigned long node,
 					   const char *uname,