WEXITSTATUS is defined in sys/wait.h

Fix compiling with dietlibc:

../libkmod/libkmod-module.c:858:2: warning: implicit declaration of function 'WEXITSTATUS' [-Wimplicit-function-declaration]
../libkmod/libkmod-module.c:858:2: warning: nested extern declaration of 'WEXITSTATUS' [-Wnested-externs]
(...)
kmod-modprobe.c:(.text.command_do+0x157): undefined reference to `WEXITSTATUS'
diff --git a/tools/kmod-modprobe.c b/tools/kmod-modprobe.c
index 9d612dd..4052d3b 100644
--- a/tools/kmod-modprobe.c
+++ b/tools/kmod-modprobe.c
@@ -26,6 +26,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/utsname.h>
+#include <sys/wait.h>
 #include <unistd.h>
 #include <syslog.h>
 #include <limits.h>