off64_t is not provided without defining _LARGEFILE64_SOURCE on musl
this define is not defined automatically like glibc where it gets
defined when _GNU_SOURCE is defined. Using off_t makes it portable
across musl/glibc and for using 64bit off_t on glibc 32bit systems
-D_FILE_OFFSET_BITS=64 can be defined during build via CXXFLAGS

Signed-off-by: Khem Raj <raj.khem@gmail.com>