maint: remove now-unnecessary #if HAVE_header_H tests.

maint: remove now-unnecessary #if HAVE_header_H tests.

* .x-sc_prohibit_always_true_header_tests: New file.
* Makefile.am (syntax_check_exceptions): Add it.
* src/cat.c: Remove #if HAVE_SYS_IOCTL_H test.
* src/copy.c: Likewise.
* src/ls.c: Likewise.
* src/stty.c: Likewise.
* src/install.c: Remove #if HAVE_SYS_WAIT_H test.
* src/kill.c: Likewise.
* src/operand2sig.c: Likewise.
* src/timeout.c: Likewise.
* src/pathchk.c: Remove #if HAVE_WCHAR_H test.
* src/stat.c: Remove #if HAVE_NETINET_IN_H test.