mpg123: fix build with msvc

mpg123: fix build with msvc

Fix syntax errors when compiling against cerbero-provided libmpg123
headers. We do the same as the libmpg123 internal visual studio
build here.

mpg123.h(1378): error C2143: syntax error: missing ')' before '('
mpg123.h(1378): error C2081: 'ssize_t': name in formal parameter list illegal
mpg123.h(1378): error C2143: syntax error: missing ')' before '*'
mpg123.h(1378): error C2091: function returns function
mpg123.h(1378): error C2143: syntax error: missing '{' before '*'
mpg123.h(1378): error C2059: syntax error: ')'
mpg123.h(1379): error C2143: syntax error: missing ')' before '*'
mpg123.h(1379): error C2365: 'off_t': redefinition; previous definition was 'typedef'
...