Fix up external def that causes FTBFS with gcc5
--- a/src/drv_pop.c
+++ b/src/drv_pop.c
@@ -91,8 +91,6 @@
 #define POP_AUTH_USER	1
 #define POP_AUTH_APOP	2
 
-char *strndup(const char *s, size_t n); 
-
 typedef struct
 {
 	long mess_num;