Fix selftest::test_lexer_string_locations_ebcdic for systems without iconv (PR bootst...

Fix selftest::test_lexer_string_locations_ebcdic for systems without iconv (PR bootstrap/72844)

selftest::test_lexer_string_locations_ebcdic has this clause:

  /* EBCDIC support requires iconv.  */
  if (!HAVE_ICONV)
    return;

leading to a build failure on systems without iconv.  This conditional
works in libcpp due to this in libcpp/internal.h:

  #if HAVE_ICONV
  #include <iconv.h>
  #else
  #define HAVE_ICONV 0
  typedef int iconv_t;  /* dummy */
  #endif

Fix the problem by ensuring that HAVE_ICONV is always defined within
gcc/input.c.

gcc/ChangeLog:
	PR bootstrap/72844
	* input.c: Ensure that HAVE_ICONV is defined.

From-SVN: r239257