Allow use of strncpy()

There are cases where strncpy() does exactly what is required.
A blanket ban forces more convoluted solutions to be used in those
cases and has been a cause of bugs.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/internal.h

@@ -70,8 +70,6 @@
 #define sprintf sprintf_is_forbidden_due_to_security_issues_use_snprintf
 #undef  strcat
 #define strcat strcat_is_forbidden_due_to_security_issues_use_av_strlcat
-#undef  strncpy
-#define strncpy strncpy_is_forbidden_due_to_security_issues_use_av_strlcpy
 #undef  exit
 #define exit exit_is_forbidden
 #undef  printf