mpi/pmix: Fix a typo in I/O engine code

Replase "bool" with "int" as the return type of `pmixp_io_fd()`. This
was causing an interesting hidden bug affecting the performance.
Since this function was returning boolean value it was always
returning "1" instead of actual fd number. fd=1 is set to /dev/null
for slurmstepd, this is a char device that is always read/write ready
from the poll() perspective. So poll was continuously interrupting and
progress was fine, but CPU usage was ~100%.

src/plugins/mpi/pmix/pmixp_io.h

@@ -77,7 +77,7 @@ typedef struct {
 	List send_queue, complete_queue;
 } pmixp_io_engine_t;
 
-static inline bool pmixp_io_fd(pmixp_io_engine_t *eng)
+static inline int pmixp_io_fd(pmixp_io_engine_t *eng)
 {
 	return eng->sd;
 }