util: move #include out of #if linux

This #include is needed for `NULL`, which is used on all OSes, not just Linux.

Reported-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Fixes: 316964709e "util: add os_read_file() helper"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>

src/util/os_file.c

@@ -6,11 +6,11 @@
 #include "os_file.h"
 
 #include <errno.h>
+#include <stdlib.h>
 
 #if defined(__linux__)
 
 #include <fcntl.h>
-#include <stdlib.h>
 #include <sys/stat.h>
 #include <unistd.h>