o_fopen: fix coding style and build error with VS2010 #56

Job	Run time
compiler (gcc-7, ubuntu-20.04)	4m 28s
compiler (gcc-8, ubuntu-20.04)	4m 24s
compiler (gcc-9, ubuntu-20.04)	4m 35s
compiler (gcc-10, ubuntu-20.04)	4m 30s
compiler (gcc-11, ubuntu-22.04)	4m 44s
compiler (gcc-12, ubuntu-22.04)	5m 10s
compiler (gcc-13, ubuntu-22.04, ubuntu-toolchain-r/test)	5m 12s
compiler (clang-6.0, ubuntu-20.04)	4m 47s
compiler (clang-7, ubuntu-20.04)	4m 47s
compiler (clang-8, ubuntu-20.04)	4m 55s
compiler (clang-9, ubuntu-20.04)	5m 14s
compiler (clang-10, ubuntu-20.04)	4m 51s
compiler (clang-11, ubuntu-20.04)	4m 53s
compiler (clang-12, ubuntu-20.04)	4m 42s
compiler (clang-13, ubuntu-22.04)	5m 8s
compiler (clang-14, ubuntu-22.04)	4m 58s
compiler (clang-15, ubuntu-22.04, jammy)	4m 59s
compiler (clang-16, ubuntu-22.04, jammy)	5m 6s
compiler (clang-17, ubuntu-22.04, jammy)	5m 8s
	1h 32m 31s

Provide feedback