[zx] Make all zx::interrupt methods const

Only C++ const-correctness is important, not whether the underlying
kernel object is mutated.

Test: Ran system/utest/core/interrupt.
Change-Id: I72529d61063ce2c360e6b13c429902a521e2133a
diff --git a/system/ulib/zx/include/lib/zx/interrupt.h b/system/ulib/zx/include/lib/zx/interrupt.h
index 2f559ae..4f4395f 100644
--- a/system/ulib/zx/include/lib/zx/interrupt.h
+++ b/system/ulib/zx/include/lib/zx/interrupt.h
@@ -32,29 +32,30 @@
     }
 
     static zx_status_t create(const resource& resource, uint32_t vector,
-                        uint32_t options, interrupt* result);
+                              uint32_t options, interrupt* result);
 
-    zx_status_t wait(zx::time* timestamp) {
+    zx_status_t wait(zx::time* timestamp) const {
         return zx_interrupt_wait(get(), timestamp->get_address());
     }
 
-    zx_status_t destroy() {
+    zx_status_t destroy() const {
         return zx_interrupt_destroy(get());
     }
 
-    zx_status_t trigger(uint32_t options, zx::time timestamp) {
+    zx_status_t trigger(uint32_t options, zx::time timestamp) const {
         return zx_interrupt_trigger(get(), options, timestamp.get());
     }
 
-    zx_status_t bind(const zx::port& port, uint64_t key, uint32_t options) {
+    zx_status_t bind(const zx::port& port, uint64_t key,
+                     uint32_t options) const {
         return zx_interrupt_bind(get(), port.get(), key, options);
     }
 
-    zx_status_t bind_vcpu(const zx::vcpu& vcpu, uint32_t options) {
+    zx_status_t bind_vcpu(const zx::vcpu& vcpu, uint32_t options) const {
         return zx_interrupt_bind_vcpu(get(), vcpu.get(), options);
     }
 
-    zx_status_t ack() {
+    zx_status_t ack() const {
         return zx_interrupt_ack(get());
     }
 };