Explicitly include header.

* ppm_extents.h is pulled in by ppm_uniform_partition.h but since the code
  contains variables of type PPM_extent, it is more consistent to include
  it explicitly.

src/pio_server.c

@@ -15,6 +15,7 @@
 #include <stdio.h>
 
 #ifdef HAVE_PARALLEL_NC4
+#include <core/ppm_extents.h>
 #include <core/ppm_combinatorics.h>
 #include <core/ppm_rectilinear.h>
 #include <ppm/ppm_uniform_partition.h>