Speculative fix for PATH_MAX not known

The latest poky master build is failing with the error below. Let's
include limits.h and see if this is already fixing it.

| rtl8168-eeprom.c: In function 'eeprom_backup':
| rtl8168-eeprom.c:406:13: error: 'PATH_MAX' undeclared (first use in this function)
|   char fname[PATH_MAX];
|              ^~~~~~~~

rtl8168-eeprom.c

@@ -4,6 +4,7 @@
 #include <unistd.h>
 #include <stdarg.h>
 #include <string.h>
+#include <limits.h>
 
 #include <pci/pci.h>
 #include <sys/fcntl.h>