target/ppc/cpu.h: Put macro parameter in parentheses

Fix PPC_INPUT macro to work with more complex expressions by
protecting its argument with parentheses.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <20200130021619.65FAB747871@zero.eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/cpu.h

@@ -180,7 +180,7 @@ enum {
     POWERPC_EXCP_TRAP          = 0x40,
 };
 
-#define PPC_INPUT(env) (env->bus_model)
+#define PPC_INPUT(env) ((env)->bus_model)
 
 /*****************************************************************************/
 typedef struct opc_handler_t opc_handler_t;