Remove ATTRIBUTE_NONNULL(3) from qemuMonitorJSONDrivePivot

The header definition didn't match the function declaration, so adjusted
header to reflect the definition.

Found during a Coverity build where STATIC_ANALYSIS is enabled resulting
in the internal.h adding __nonnull__ handling to arguments.

Commit '6d264c91' added support for the qemuMonitorJSONDrivePivot() and
commit 'fbc3adc9' added a corresponding test which ended up triggering
the build failure which I didn't notice until today!

src/qemu/qemu_monitor_json.h

@@ -250,7 +250,7 @@ int qemuMonitorJSONDrivePivot(qemuMonitorPtr mon,
                               const char *device,
                               const char *file,
                               const char *format)
-    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) ATTRIBUTE_NONNULL(3);
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 int qemuMonitorJSONBlockCommit(qemuMonitorPtr mon,
                                const char *device,