build fails on musl-libc based linux systems (for example alpine, void, sabotage)

Created by: rofl0r

dynamorio-release_6_1_1/core/unix/os.c: In function 'handle_close_pre':
dynamorio-release_6_1_1/core/unix/os.c:5620:30: error: dereferencing pointer to incomplete type
dynamorio-release_6_1_1/core/unix/os.c:5636:30: error: dereferencing pointer to incomplete type
dynamorio-release_6_1_1/core/unix/os.c:5652:29: error: dereferencing pointer to incomplete type
make[2]: *** [core/CMakeFiles/dynamorio.dir/unix/os.c.o] Error 1

the code in question is

            /* update the privately loaded libc's stdout _fileno. */
            (*privmod_stdout)->STDFILE_FILENO = our_stdout;                     


            /* update the privately loaded libc's stderr _fileno. */            
            (*privmod_stderr)->STDFILE_FILENO = our_stderr;


            /* update the privately loaded libc's stdout _fileno. */            
            (*privmod_stdin)->STDFILE_FILENO = our_stdin;

FILE is an opaque type and it's illegal to make assumptions about or access the underlying struct from non-libc code. I'm currently talking with musl's author if there's a portable solution. the code in question was introduced to fix #261