[Bug driver/126506] New: gcc.cc: Only define add_sysrooted_hdrs_prefix if NATIVE_SYSTEM_HEADER_DIR is defined

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126506

            Bug ID: 126506
           Summary: gcc.cc: Only define add_sysrooted_hdrs_prefix if
                    NATIVE_SYSTEM_HEADER_DIR is defined
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dragon-archer at outlook dot com
  Target Milestone: ---

`add_sysrooted_hdrs_prefix` is only used in `find_fortran_preinclude_file` when
NATIVE_SYSTEM_HEADER_DIR is defined, while itself is defined unconditionally.
This generates the following warning when compiling GCC with
NATIVE_SYSTEM_HEADER_DIR not defined (e.g. on MinGW-w64):

../../gcc-16.1.0/gcc/gcc.cc:3231:1: warning: 'void
add_sysrooted_hdrs_prefix(path_prefix*, const char*, const char*, int, int,
int)' defined but not used [-Wunused-function]
 3231 | add_sysrooted_hdrs_prefix (struct path_prefix *pprefix, const char
*prefix,
      | ^~~~~~~~~~~~~~~~~~~~~~~~~