community/sdcv: fix build with gcc 14

Fix invalid conversion error with gcc 14 (rebuild error log). Includes some minor adjustments for lint.

/home/buildozer/aports/community/sdcv/src/sdcv-0.5.5/src/stardict_lib.cpp:
In member function 'bool Libs::LookupSimilarWord(const gchar*, std::set<long int>&, int)':
/usr/include/glib-2.0/glib/gunicode.h:809:34: error: invalid conversion from 'const gchar*' {aka 'const char*'} to 'gchar*' {aka 'char*'} [-fpermissive]
  809 | #define g_utf8_next_char(p) ((p) + g_utf8_skip[*(const guchar *)(p)])
      |                             ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                  |
      |                                  const gchar* {aka const char*}
/home/buildozer/aports/community/sdcv/src/sdcv-0.5.5/src/stardict_lib.cpp:1050:31: note: in expansion of macro 'g_utf8_next_char'
 1050 |             gchar *nextchar = g_utf8_next_char(sWord);
      |                               ^~~~~~~~~~~~~~~~