BUILD: debug: silence warning on gcc-5

In 2.6, 8a0fd3a36 ("BUILD: debug: work around gcc-12 excessive
-Warray-bounds warnings") disabled some warnings that were reported
around the the BUG() statement. But the -Wnull-dereference warning
isn't known from gcc-5, it only arrived in gcc-6, hence makes gcc-5
complain loudly that it doesn't know this directive. Let's just
condition this one to gcc-6.

include/haproxy/bug.h

@@ -45,7 +45,9 @@ static inline __attribute((always_inline)) void ha_crash_now(void)
 #if __GNUC_PREREQ__(5, 0)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
+#if __GNUC_PREREQ__(6, 0)
 #pragma GCC diagnostic ignored "-Wnull-dereference"
+#endif
 #endif
 	*(volatile char *)1 = 0;
 #if __GNUC_PREREQ__(5, 0)