extrawarn: enable more warnings in W=2

These four warning options are always disabled, but actually meet the
criteria for W=2, as they are sometimes useful and not prohibitively
noisy:

 -Wformat-security
 -Wframe-address
 -Waddress-of-packed-member
 -Wtrigraphs

Signed-off-by: Arnd Bergmann <arnd@arndb.de>