Added two missing headers that raised warning messages with the Intel compiler.

Former-commit-id: 715f950ec856c778b0752f28763d706f896a4906

src/runner.c

@@ -23,6 +23,7 @@
 /* Some standard headers. */
 #include <float.h>
 #include <limits.h>
+#include <stdlib.h>
 
 /* MPI headers. */
 #ifdef WITH_MPI

src/space.c

@@ -25,6 +25,7 @@
 #include <limits.h>
 #include <math.h>
 #include <string.h>
+#include <stdlib.h>
 
 /* MPI headers. */
 #ifdef WITH_MPI