Minor fix for GCC 5.2

The newer GCC versions are more strict about signed/unsigned correctness and raise warnings.

src/space.c

@@ -492,8 +492,8 @@ void parts_sort(struct part *parts, struct xpart *xparts, int *ind, int N,
unsigned int qstack_size = 2 * (max - min) + 10;
volatile unsigned int first, last, waiting;
-  int pivot;
-  int i, ii, j, jj, temp_i, qid;
+  unsigned int pivot;
+  unsigned int i, ii, j, jj, temp_i, qid;
struct part temp_p;
struct xpart temp_xp;
@@ -636,8 +636,8 @@ void gparts_sort(struct gpart *gparts, int *ind, int N, int min, int max) {
int qstack_size = 2 * (max - min) + 10;
volatile unsigned int first, last, waiting;
-  int pivot;
-  int i, ii, j, jj, temp_i, qid;
+  unsigned int pivot;
+  unsigned int i, ii, j, jj, temp_i, qid;
struct gpart temp_p;
/* for ( int k = 0 ; k < N ; k++ )