banned.h: mark strncat() as banned

strncat() has the same quadratic behavior as strcat() and is
difficult-to-read and bug-prone.  While it hasn't yet been a
problem in git iself, strncat() found it's way into 'master'
of cgit and caused segfaults on my system.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

banned.h

@@ -16,6 +16,8 @@
 #define strcat(x,y) BANNED(strcat)
 #undef strncpy
 #define strncpy(x,y,n) BANNED(strncpy)
+#undef strncat
+#define strncat(x,y,n) BANNED(strncat)
 
 #undef sprintf
 #undef vsprintf