fix compilation with stderr trace backend

Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

trace-events

@@ -455,7 +455,7 @@ milkymist_vgafb_memory_write(uint32_t addr, uint32_t value) "addr %08x value %08
 mipsnet_send(uint32_t size) "sending len=%u"
 mipsnet_receive(uint32_t size) "receiving len=%u"
 mipsnet_read(uint64_t addr, uint32_t val) "read addr=0x%" PRIx64 " val=0x%x"
-mipsnet_write(uint64_t addr, uint64_t val) "write addr=0x%" PRIx64 " val=0x%" PRIx64
+mipsnet_write(uint64_t addr, uint64_t val) "write addr=0x%" PRIx64 " val=0x%" PRIx64 ""
 mipsnet_irq(uint32_t isr, uint32_t intctl) "set irq to %d (%02x)"
 
 # xen-all.c