clang: suppress a warning

Define missing _GNU_SOURCE for mkostemp().

    lib/ii.c:7217:30: warning: implicit declaration
          of function 'mkostemp' is invalid in C99 [-Wimplicit-function-declaration]
              ii_buffer->tmpfd = GRN_MKOSTEMP(ii_buffer->tmpfpath,
                                 ^
    lib/groonga_in.h:523:45: note: expanded from
          macro 'GRN_MKOSTEMP'
    #  define GRN_MKOSTEMP(template,flags,mode) mkostemp(template,flags)
                                                ^