remove recapitulated prototypes of strcasecmp

remove recapitulated prototypes of strcasecmp

These actually cause warnings in CMake Windows builds:

  …\lib\cgraph/strcasecmp.h(15,47): warning C4273: '_stricmp': inconsistent
    dll linkage […\graphviz\build\plugin\gd\gvplugin_gd.vcxproj]
  C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\ucrt\string.h(193,22):
    message : see previous definition of '_stricmp'
    […\build\plugin\gd\gvplugin_gd.vcxproj]

and MSBuild Windows builds:

  …\lib\cgraph\strcasecmp.h(23,64): warning C4211: nonstandard extension used:
    redefined extern to static […\graphviz\lib\cgraph\cgraph.vcxproj]
  …\lib\cgraph\strcasecmp.h(27,75): warning C4211: nonstandard extension used:
    redefined extern to static […\lib\cgraph\cgraph.vcxproj]

and Cygwin MinGW builds:

  …/lib/cgraph/strcasecmp.h:15:5: warning: '_stricmp' redeclared without
    dllimport attribute: previous dllimport ignored [-Wattributes]
     15 | int strcasecmp(const char *s1, const char *s2);
        |     ^~~~~~~~~~
  …/lib/cgraph/strcasecmp.h:16:5: warning: '_strnicmp' redeclared without
    dllimport attribute: previous dllimport ignored [-Wattributes]
     16 | int strncasecmp(const char *s1, const char *s2, size_t n);
        |     ^~~~~~~~~~~

Related to #1940.