o_fopen: fix coding style and build error with VS2010

o_fopen: fix coding style and build error with VS2010

Follow the coding style to place variable definitions before code

Fixes a build error on Windows 2003 with VS2010 introduced in [1]
crypto\o_fopen.c(45) : error C2143: syntax error : missing ';' before 'type'
crypto\o_fopen.c(46) : error C2275: 'DWORD' : illegal use of this type as an expression
        E:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\include\windef.h(152) : see declaration of 'DWORD'
crypto\o_fopen.c(46) : error C2146: syntax error : missing ';' before identifier 'flags'
crypto\o_fopen.c(46) : error C2065: 'flags' : undeclared identifier

[1] https://github.com/openssl/openssl/commit/917f37195ac95252a4c90e86d7d7414c5569aed8

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24853)