Remove duplicate #include directives.

Remove duplicate #include directives.

* alloc.c [GC_MARK_STACK == GC_USE_GCPROS_CHECK_ZOMBIES]:
* xfaces.c:
Don't include stdio.h twice.
* buffer.c [USE_MMAP_FOR_BUFFERS]:
Don't include sys/types.h or stdio.h twice.
* fileio.c [WINDOWSNT | MSDOS]: Don't include fcntl.h twice.
* lread.c: Don't include coding.h twice.
* nsfont.m: Don't include frame.h twice.
* process.c [HAVE_RES_INIT]: Don't include <netinet/in.h> twice.
* ralloc.c: Don't include <unistd.h> twice.
* xdisp.c: Don't include font.h twice.
* xterm.c: Don't include fontset.h twice.
* xterm.h [USE_X_TOOLKIT]: Don't include X11/StringDefs.h twice.