Add include files for prototypes

Include the header with prototypes in the file that provides definitions
as well, to catch any mismatch between prototype and definition.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10470

lib/libspl/os/linux/getexecname.c

@@ -25,10 +25,11 @@
  */
 
 
+#include <limits.h>
+#include <pthread.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
-#include <pthread.h>
-#include <limits.h>
 
 const char *
 getexecname(void)