package/rsyslog: fix musl buil

The build error was not yet being found by the autobuilders:

omfile.c: In function ‘prepareFile’:
omfile.c:580:3: error: implicit declaration of function ‘open’ [-Werror=implicit-function-declaration]
   fd = open((char*) newFileName, O_WRONLY|O_APPEND|O_CREAT|O_NOCTTY|O_CLOEXEC,
   ^

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>