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!