Adds some more missing POSIX API definitions (#1581)

* Adds missing POSIX API definition for `strchrnul`

* `__stack_chk_fail`

Author: bmourad01

plugins/api/api/c/gnu.h

@@ -1 +1,2 @@
 int __libc_start_main(int (*main) (int, char **, char **), int, char **, void *auxv);
+void __stack_chk_fail(void) __attribute__((noreturn));

plugins/api/api/c/posix.h

@@ -205,6 +205,9 @@ char *strcat(char * restrict dst, const char * restrict src)
 char *strchr(const char *s, int c)
     __attribute__((pure,nonnull(1),storage(1)));
 
+char *strchrnul(const char *s, int c)
+    __attribute__((pure,nonnull(1),storage(1)));
+
 int strcmp(const char *s1, const char *s2)
     __attribute__((pure, nonnull(1,2), storage(1), storage(2)));