detect: assist clang to suppress warning

detect: assist clang to suppress warning

  CC       detect-engine-loader.o
In file included from /usr/include/stdio.h:970,
                 from suricata-common.h:77,
                 from detect-engine-loader.c:24:
In function 'fgets',
    inlined from 'DetectLoadSigFile' at detect-engine-loader.c:139:11:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:313:12: warning: argument 2 value -1 is negative [-Wstringop-overflow=]
  313 |     return __fgets_alias (__s, __n, __stream);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/features.h:523,
                 from /usr/include/dirent.h:25,
                 from suricata-common.h:73:
/usr/include/x86_64-linux-gnu/bits/stdio2-decl.h: In function 'DetectLoadSigFile':
/usr/include/x86_64-linux-gnu/bits/stdio2-decl.h:96:14: note: in a call to function '__fgets_alias' declared with attribute 'access (write_only, 1, 2)'
   96 | extern char *__REDIRECT (__fgets_alias,
      |              ^~~~~~~~~~