Provide loff_t via <fcntl.h> on musl-based Linux systems

Musl exposes loff_t only as a macro in <fcntl.h> when _GNU_SOURCE is
defined. Including <fcntl.h> ensures the type is available, and a
fallback typedef is provided when no macro is defined. This fixes build
failures on musl systems.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Moch <mail@alexmoch.com>
Closes #18002