maint: use pragma consistently across replacement headers

maint: use pragma consistently across replacement headers

Several headers set #pragma GCC system_header prior to checking
any multiple inclusion guards; maintenance is easiest if all
replacement headers follow the same paradigm.

* lib/ctype.in.h (system_header): Hoist for consistent placement.
* lib/dirent.in.h (system_header): Likewise.
* lib/errno.in.h (system_header): Likewise.
* lib/float.in.h (system_header): Likewise.
* lib/getopt.in.h (system_header): Likewise.
* lib/iconv.in.h (system_header): Likewise.
* lib/inttypes.in.h (system_header): Likewise.
* lib/langinfo.in.h (system_header): Likewise.
* lib/locale.in.h (system_header): Likewise.
* lib/math.in.h (system_header): Likewise.
* lib/netdb.in.h (system_header): Likewise.
* lib/netinet_in.in.h (system_header): Likewise.
* lib/pty.in.h (system_header): Likewise.
* lib/sched.in.h (system_header): Likewise.
* lib/se-selinux.in.h (system_header): Likewise.
* lib/search.in.h (system_header): Likewise.
* lib/spawn.in.h (system_header): Likewise.
* lib/stdarg.in.h (system_header): Likewise.
* lib/stdint.in.h (system_header): Likewise.
* lib/string.in.h (system_header): Likewise.
* lib/strings.in.h (system_header): Likewise.
* lib/sys_file.in.h (system_header): Likewise.
* lib/sys_ioctl.in.h (system_header): Likewise.
* lib/sys_socket.in.h (system_header): Likewise.
* lib/sys_times.in.h (system_header): Likewise.
* lib/sys_utsname.in.h (system_header): Likewise.
* lib/sys_wait.in.h (system_header): Likewise.
* lib/sysexits.in.h (system_header): Likewise.
* lib/unistd.in.h (system_header): Likewise.
* lib/wctype.in.h (system_header): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>