More forceful clearing of stack memory with keys

gcc 8.3.0 was apparently clever enough to optimize away the previously
used os_memset() to explicitly clear a stack buffer that contains keys
when that clearing happened just before returning from the function.
Since memset_s() is not exactly portable (or commonly available yet..),
use a less robust mechanism that is still pretty likely to prevent
current compilers from optimizing the explicit clearing of the memory
away.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/utils/common.h

@@ -570,6 +570,8 @@ int str_starts(const char *str, const char *start);
 u8 rssi_to_rcpi(int rssi);
 char * get_param(const char *cmd, const char *param);
 
+void forced_memzero(void *ptr, size_t len);
+
 /*
  * gcc 4.4 ends up generating strict-aliasing warnings about some very common
  * networking socket uses that do not really result in a real problem and